Selina Concise Solutions for ICSE Class 10 Physics Chapter 5 Refraction Through A Lens

Exercise 5(A)

Question 1. What is a lens?
Answer: A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical.
In simple words: A lens is a clear piece of glass or plastic. It has two curved sides. Light bends when it passes through the lens.

📝 Teacher's Note: Show students a magnifying glass or spectacles. The curved glass piece is a lens. Light bends when it goes through the curved surface.

🎯 Exam Tip: Always write "transparent", "curved surfaces", and "spherical". These are key words examiners look for.

Question 2. Name the different kinds of lenses. Draw diagrams to illustrate them.
Answer: Lenses are of two types:

1. Convex or converging lens, and
2. Concave or diverging lens.

[Diagram: Shows two lens shapes side by side - a convex lens that is thick in the middle and thin at edges, and a concave lens that is thin in the middle and thick at edges.]

In simple words: There are two types of lenses. Convex lens is fat in the middle. Concave lens is thin in the middle.

📝 Teacher's Note: Use your hands to show the shapes. Make a fat middle with fingers for convex. Make a thin middle for concave.

🎯 Exam Tip: Always write both names - "convex or converging" and "concave or diverging". Draw clear diagrams with labels.

Question 3. State two differences between a convex and a concave lens.
Answer: Convex lens:

1. It converge the incident rays towards the principal axis.
2. It has a real focus.

Concave lens:

1. It diverges the incident rays away from the principal axis.
2. It has a virtual focus.

In simple words: Convex lens brings light rays together (converge). Concave lens spreads light rays apart (diverge). Real focus means you can see it on a screen. Virtual focus means you cannot see it on a screen.

📝 Teacher's Note: Use a torch and lens to show how convex brings light together and concave spreads it apart.

🎯 Exam Tip: Write "converge" for convex and "diverge" for concave. Also write "real focus" and "virtual focus" clearly.

Question 4. Which lens is converging: An equiconcave lens or an equiconvex lens.
Answer: Equiconvex lens is converging.
In simple words: Equiconvex means both sides of the lens curve outwards equally. This type brings light rays together, so it is converging.

📝 Teacher's Note: Explain "equi" means equal. So equiconvex has equal curves on both sides. All convex lenses converge light.

🎯 Exam Tip: Remember - convex always converges. The word "equiconvex" still has "convex" in it.

Question 5. Out of the two lenses, one concave and the other convex, state which one will show the divergent action on a light beam.
Answer: Concave lens will show the divergent action on a light beam.
In simple words: Concave lens spreads light rays apart. This spreading action is called divergent action.

📝 Teacher's Note: Remember the simple rule - concave diverges, convex converges. Use hand gestures to show spreading apart and coming together.

🎯 Exam Tip: Write "concave lens" clearly. The word "divergent" is the key word examiners want to see.

Question 6. Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass block and two triangular glass prisms.
Answer: As shown in the figure the convex lens has two glass prisms and one glass block. One of the glass prisms is situated above the glass block and one below the block.

[Diagram: Shows a convex lens broken down into three parts - an upper triangular prism, a middle rectangular glass block, and a lower triangular prism. Light rays A, B, and C are shown entering parallel and converging to point F after refraction.]

In simple words: A convex lens works like three pieces joined together. The top and bottom parts bend light inward. The middle part lets light pass straight.

📝 Teacher's Note: Show students how a lens can be thought of as prisms stacked together. The prisms at top and bottom do most of the bending.

🎯 Exam Tip: Draw the diagram clearly with labels A, B, C for rays. Show the three parts - two prisms and one block.

Question 7. Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a concave lens by treating it as a combination of a glass block and two triangular glass prisms.
Answer: As shown in the figure the concave lens has two glass prisms and one glass block. One of the glass prisms is situated above the glass block and one below the block.

[Diagram: Shows a concave lens broken down into three parts with light rays A, B, and C entering parallel and diverging away from the principal axis after refraction, appearing to come from virtual focus point F.]

In simple words: A concave lens also works like three pieces. But here the prisms bend light outward, spreading the rays apart.

📝 Teacher's Note: Compare this with the convex lens diagram. Show how the prism arrangement is different and causes spreading instead of converging.

🎯 Exam Tip: Draw rays diverging (spreading apart) for concave lens. Mark the virtual focus point F with dotted lines.

Question 8. How does the action of a convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagram to illustrate your answer.
Answer: If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the incident ray downwards. The lower part bens the ray upwards while the central part passes the ray undeviated. But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower part bends the ray downwards while the central part passes the ray undeviated.

[Diagram: Shows two diagrams side by side - convex lens converging parallel rays to focus point F2, and concave lens diverging parallel rays appearing to come from virtual focus F1.]

In simple words: Convex lens brings all rays to one point (converges). Concave lens spreads rays apart (diverges). The middle ray goes straight in both cases.

📝 Teacher's Note: Use torch light and both types of lenses to demonstrate this difference. Students can see the converging and diverging action clearly.

🎯 Exam Tip: Draw both diagrams side by side for comparison. Label focus points clearly as F2 for convex and F1 for concave.

Question 9. Define the term principal axis of a lens.
Answer: It is the line joining the centers of curvature of the two surfaces of the lens.
In simple words: Principal axis is an imaginary straight line that goes through the middle of the lens. It connects the centers of the two curved surfaces.

📝 Teacher's Note: Draw a lens and show the two curved surfaces. Mark their centers and draw a line connecting them. This line is the principal axis.

🎯 Exam Tip: Write "line joining centers of curvature" exactly. This is the definition examiners expect.

Question 10. Explain optical centre of a lens with the help of proper diagram(s)
Answer: It is point on the principal axis of the lens such that a ray of light passing through this point emerges parallel to its direction of incidence. It is marked by letter O in the figure. The optical centre is thus the centre of the lens.

[Diagram: Shows a lens with a ray passing through point O (optical centre) and emerging parallel to its original direction.]

In simple words: Optical centre is a special point in the middle of the lens. Any light ray passing through this point comes out in the same direction it went in.

📝 Teacher's Note: Show students that this is like the center of the lens. Light passing through the center does not bend - it goes straight.

🎯 Exam Tip: Write "emerges parallel to direction of incidence" and "center of the lens". Mark point O clearly in your diagram.

Question 11. A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens. (a) What special name is given to this point on the principal axis? (b) Draw a labelled diagram to support your answer in part (a)
Answer:
(a) This point is known as Optical centre.
(b)

[Diagram: Shows a convex lens with point O marked as optical centre, and a light ray passing through O without bending.]

In simple words: The special point where light does not bend is called optical centre. It is the center point of the lens.

📝 Teacher's Note: Emphasize that this is the only point where light passes without bending. All other points cause refraction.

🎯 Exam Tip: Write "Optical centre" with capital letters. Draw the ray going straight through point O without any bending.

Question 12. State the condition when a lens is called an equi-convex or equi-concave
Answer: A lens is called an equiconvex or equiconcave when radii of curvature of the two surfaces of lens are equal.
In simple words: When both curved sides of the lens have the same amount of curve, we call it equi-convex or equi-concave. "Equi" means equal.

📝 Teacher's Note: Show students lenses with equal curves on both sides. Compare with lenses that have different curves on each side.

🎯 Exam Tip: Write "radii of curvature are equal" exactly. This is the key condition examiners look for.

Question 13. Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.
Answer: A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. For a convex lens, the first focal point is a point F1 on the principal axis of the lens such that the rays of light starting from it or passing through it, after refraction through lens, become parallel to the principal axis of the lens. The second focal point for a convex lens is a point F2 on the principal axis such that the rays of light incident parallel to the principal axis, after refraction from the lens, pass through it.

[Diagram: Shows two diagrams - first showing rays from F1 becoming parallel after refraction, second showing parallel rays converging to F2 after refraction.]

In simple words: A convex lens has two focus points called F1 and F2. F1 is where rays start from to become parallel. F2 is where parallel rays meet after passing through the lens.

📝 Teacher's Note: Use a magnifying glass and sunlight to show students how parallel rays meet at the focus point. This makes the concept very clear.

🎯 Exam Tip: Always mention "two principal foci" and label them as F1 and F2. Draw both diagrams to show both focus points.

Question 14. Define the term principal foci of a concave lens and show them with the help of proper diagrams.
Answer: A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. For a concave lens, the first focal point is a point F1 on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens. The second focal point for a concave lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.

[Diagram: Shows a concave lens with parallel rays on the principal axis appearing to diverge from virtual focus point F1, and rays appearing to meet at F2 after refraction.]

In simple words: A concave lens also has two focus points F1 and F2. But these are virtual focus points. The rays appear to come from these points but do not actually meet there.

📝 Teacher's Note: Explain "virtual" means the rays appear to come from the focus but do not actually meet there. Use dotted lines to show this in diagrams.

🎯 Exam Tip: Write "appear to meet" and "appear to be diverging" for concave lens. Use dotted lines to show virtual rays in your diagrams.

Question 15. Draw a diagram to represent the second focus of a concave lens.
Answer:
[Diagram: This diagram shows a concave lens with parallel rays coming from the right side. These rays diverge after passing through the lens, and when extended backwards, they appear to meet at a point F₂ on the left side of the lens. This point F₂ is the second focus of the concave lens.]
In simple words: The second focus is the point where light rays seem to come from when parallel rays hit a concave lens. The rays spread out, but if we draw them backwards, they meet at this focus point.

📝 Teacher's Note: Show students how to draw dotted lines backwards from the diverging rays. This helps them see where the rays would meet if extended. Use a torch and concave lens to show this in class.

🎯 Exam Tip: Always draw dotted lines to show the backward extension of rays. Label the focus point clearly as F₂. This shows you understand virtual focus.

Question 16. Draw a diagram to represent the second focus of a convex lens.
Answer:
[Diagram: This diagram shows a convex lens with parallel rays coming from the left side. These rays converge after passing through the lens and meet at a point F₂ on the right side of the lens. This point F₂ is the second focus of the convex lens.]
In simple words: The second focus is the real point where all parallel light rays meet after passing through a convex lens. The rays actually come together at this point.

📝 Teacher's Note: Use a magnifying glass and sunlight to show students how parallel rays from the sun meet at one bright point. That point is the second focus. Be careful with the bright spot - it can burn!

🎯 Exam Tip: Draw solid lines for the converged rays because they actually meet. Label the focus as F₂. Show that rays are parallel before hitting the lens.

Question 17. A ray of light, after refraction through a concave lens emerges parallel to the principal axis. (a) draw a ray diagram to show the incident ray and its corresponding emergent ray. (b) The incident ray when produces meets the principal axis at a point. Name the point.
Answer:
(a)
[Diagram: This diagram shows a concave lens with an incident ray that, when extended backwards, passes through point F₁ on the principal axis. After refraction through the lens, this ray emerges parallel to the principal axis.]
(b) The point where incident ray when produced meets the principal axis is called first focus.
In simple words: When light comes out of a concave lens parallel to the main line, it means the light was coming towards the first focus point before hitting the lens.

📝 Teacher's Note: Tell students this is the reverse of what happens with the second focus. If we put a light source at F₁, the rays will come out parallel. Draw the backward extension line clearly.

🎯 Exam Tip: Always extend the incident ray backwards with a dotted line to show it passes through F₁. Write "first focus" clearly for part (b).

Question 18. A ray of light after refraction through a convex lens emerges parallel to principal axis. (a) Draw a ray diagram to show it. (b) The incident ray passes through a point on the principal axis. Name the point.
Answer:
(a)
[Diagram: This diagram shows a convex lens with an incident ray that passes through point F₁ on the principal axis before hitting the lens. After refraction through the lens, this ray emerges parallel to the principal axis.]
(b) The point where incident ray passes through a point on the principal axis is called first focus.
In simple words: When light passes through the first focus of a convex lens, it comes out parallel to the main line on the other side.

📝 Teacher's Note: This is one of the three basic rules for convex lens. Any ray through F₁ comes out parallel. Use a laser pointer through F₁ to show this clearly to students.

🎯 Exam Tip: Draw the incident ray actually passing through F₁ (not extended like in concave lens). The emergent ray must be perfectly parallel to the principal axis.

Question 19. A beam of light incident on a convex lens parallel to its principal axis converges at a point on the principal axis. Name the point. Draw a ray diagram to show it.
Answer: Such a point will be second focus.
[Diagram: This diagram shows parallel rays hitting a convex lens and converging at point F₂ on the principal axis after refraction.]
In simple words: When parallel light rays hit a convex lens, they all meet at one point called the second focus. This is how magnifying glasses work in sunlight.

📝 Teacher's Note: This is the most basic property of convex lens. Show with sunlight and magnifying glass. The bright spot where rays meet is F₂. Students remember this easily.

🎯 Exam Tip: Write "second focus" and draw the diagram showing multiple parallel rays all meeting at F₂. This is the defining property of convex lens.

Question 20. A beam of light incident on a thin concave lens parallel to its principal axis diverges and appears to come from a point on the principal axis. Name the point. Draw a ray diagram to show it.
Answer: It appears to come from 'Second Focus'.
[Diagram: This diagram shows parallel rays hitting a concave lens and diverging. When the diverged rays are extended backwards with dotted lines, they appear to meet at point F₂ on the principal axis.]
In simple words: When parallel light hits a concave lens, the rays spread out. But if we draw lines backwards, they seem to come from the second focus point.

📝 Teacher's Note: Explain that the rays don't actually meet at F₂ - they just appear to come from there. Use dotted lines to show this is virtual. Compare with convex lens where rays really meet.

🎯 Exam Tip: Use dotted lines for the backward extension. Write "appears to come from" not "meets at" because this is virtual focus for concave lens.

Question 21. Define the term focal length of a lens.
Answer: The distance from the optical centre O of the lens to its second focal point is called the focal length of the lens.
In simple words: Focal length is how far the focus point is from the middle of the lens. It tells us how strong the lens is at bending light.

📝 Teacher's Note: Show students that shorter focal length means stronger lens. A magnifying glass with short focal length bends light more than one with long focal length.

🎯 Exam Tip: Always mention "distance from optical centre to second focus." Don't just say "distance to focus" - be specific about which focus.

Question 22. What do you mean by focal plane of a lens?
Answer: A plane passing through the focal point and normal to the principal axis of the lens is called the first focal plane.
In simple words: The focal plane is like a flat sheet that passes through the focus point and is perpendicular to the main line of the lens.

📝 Teacher's Note: Draw this as a vertical line through F₁ or F₂. Explain that any point on this plane can act like a focus for rays parallel to different directions.

🎯 Exam Tip: Write "perpendicular to principal axis" clearly. Draw the focal plane as a line through the focus point at 90 degrees to the principal axis.

Question 23. State the condition for each of the following:
(i) a lens has both its focal lengths equal
(ii) a ray passes undeviated through the lens
Answer:
(i) If a lens has both its focal length equal medium is same on either side of lens.
(ii) If a ray passes undeviated through the lens it is incident at the optical centre of the lens.
In simple words: (i) Both focus distances are equal when the lens is in air on both sides. (ii) Light goes straight through without bending only when it hits the exact center of the lens.

📝 Teacher's Note: For part (i), explain that if lens is in water on one side and air on other side, the focal lengths will be different. For part (ii), show with a ruler through lens center.

🎯 Exam Tip: Write "same medium on both sides" for part (i). For part (ii), write "passes through optical centre" - this is the key phrase examiners want.

Question 24. A parallel oblique beam of light falls on a (i) convex lens, (ii) concave lens. Draw a diagram in each case to show the refraction of light through the lens.
Answer:
[Diagram: The first diagram shows parallel oblique rays hitting a convex lens. All rays converge to meet at a point on the focal plane. The second diagram shows parallel oblique rays hitting a concave lens. The rays diverge, and when extended backwards, they appear to come from a point on the focal plane.]
In simple words: When slanted parallel rays hit a convex lens, they meet at one point on the focus plane. When they hit a concave lens, they spread out but seem to come from one point on the focus plane.

📝 Teacher's Note: Explain that oblique means slanted. The rays are still parallel to each other, just not along the principal axis. They meet on the focal plane, not exactly at F₂.

🎯 Exam Tip: Draw multiple parallel rays that are slanted. Show them meeting on the focal plane (the vertical line through focus), not exactly at the focus point.

Question 25. The diagram alongside shows a lens as a combination of a glass block and two prisms.
(i) Name the lens formed by the combination.
(ii) what is the rays the line XX' called?
(iii) Complete the ray diagram and show the path of the incident ray AB after passing through the lens.
(iv) The final emergent ray will either meet XX' at a point or appear to come from a point on XX'. Label the point as F. What is this point called?
Answer:
(i) The combination forms convex lens.
(ii) XX' is known as principal axis.
(iii)
[Diagram: Shows the incident ray AB passing through the convex lens (combination of glass block and prisms) and converging to meet at point F on the principal axis XX'.]
(iv) The point F is called as Focal point or focus.
In simple words: This shape makes a convex lens. The main line through the middle is the principal axis. The ray bends and meets at the focus point.

📝 Teacher's Note: Show students that convex lens is thick in middle, thin at edges - like two prisms base-to-base with a block in middle. This helps them remember the shape.

🎯 Exam Tip: Label all parts clearly. Write "principal axis" for XX' and "focal point" or "focus" for point F. Show the ray bending towards the axis.

Question 26. The diagram alongside shows a lens as a combination of a glass block and two prisms.
(i) Name the lens formed by the combination.
(ii) what is the line XX' called?
(iii) Complete the path of the incident ray AB after passing through the lens.
(iv) The final emergent ray either meets XX' at a point or appears to come from a point on XX' Label it as F. what is this point called?
Answer:
(i) The combination forms concave lens.
(ii) XX' is known as principal axis.
(iii)
[Diagram: Shows the incident ray AB passing through the concave lens (combination of glass block and prisms) and diverging. When extended backwards, it appears to come from point F on the principal axis XX'.]
(iv) The point F is called as Focal point or focus.
In simple words: This shape makes a concave lens. The main line is still the principal axis. The ray bends away and seems to come from the focus point.

📝 Teacher's Note: Show that concave lens is thin in middle, thick at edges - like two prisms tip-to-tip with a block in middle. Opposite shape to convex lens.

🎯 Exam Tip: For concave lens, use dotted lines to show the ray appears to come from F. Write "appears to come from" not "meets at" for concave lens focus.

Question 27. In Fig. 5.17, F₁ and F₂ are the positions of the two foci of the thin lenses shown in diagram (a) and (b) draw accurately the path taken by the light ray AB after it emerges from the lens in each diagram (a) and (b).
Answer:
[Diagram: Two ray diagrams showing path of light ray AB through convex and concave lenses. In (a) convex lens, ray AB parallel to axis passes through lens and emerges through focus F₁. In (b) concave lens, ray AB parallel to axis emerges as if coming from focus F₁.]

📝 Teacher's Note: Show students that parallel rays always go to the focus in convex lens but appear to come from focus in concave lens. Use a magnifying glass to demonstrate this rule.

🎯 Exam Tip: Always draw the emergent ray neatly with arrow. In convex lens, parallel ray goes to F₁. In concave lens, parallel ray appears to come from F₁.

Question 28. In Fig 5.18, F₁ and F₂ are the two foci of the thin lenses shown in diagram (a) and (b) and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens in each diagram (a) and (b).
Answer:
[Diagram: Two ray diagrams showing incident ray AB hitting the lens. In (a) concave lens, ray AB hits the lens and emerges parallel to principal axis. In (b) convex lens, ray AB coming from focus passes through lens and emerges parallel to principal axis.]

📝 Teacher's Note: Tell students that ray coming from focus (or towards focus in concave lens) always emerges parallel to the axis. This is the reverse of the parallel ray rule.

🎯 Exam Tip: Remember: ray from focus becomes parallel ray. Draw the emergent ray exactly parallel to principal axis with proper arrow direction.

Question 29. Complete the following sentences:
(a) If half part of a convex lens is covered, the focal length ……………… change, but the intensity of image ……………….
(b) A convex lens is placed in water. Its focal length will ……………………….
(c) The focal length of a thin convex lens is …………….. than that of a thick convex lens.
Answer:
(a) If half part of a convex lens is covered, the focal length does not change, but the intensity of image decreases.
(b) A convex lens is placed in water. Its focal length will increase.
(c) The focal length of a thin convex lens is more than that of a thick convex lens.

📝 Teacher's Note: Explain that focal length depends on lens shape, not size. But covering part reduces light, so image becomes dimmer. In water, lens bends light less, so focal length increases.

🎯 Exam Tip: Write "does not change" for focal length when lens is covered. Write "increases" for focal length in water. Write "more" for thin lens focal length.

Multiple Choice Type

Question 1. A ray of light after refraction through a lens emerges parallel to the principal axis of the lens. The incident ray either passes through or appears to meet at:
(a) optical centre
(b) first focus
(c) second focus
(d) centre of curvature of the first surface
Answer: (b) first focus
In simple words: When light comes from the focus and passes through the lens, it becomes parallel to the axis. This is a basic lens rule.

📝 Teacher's Note: Show students with a magnifying glass that when you point it towards the sun (focus), all rays become parallel. This helps them remember the rule easily.

🎯 Exam Tip: Remember the rule: ray from first focus becomes parallel ray. Always choose "first focus" for this type of question.

Question 2. A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from:
(a) its first focus
(b) its optical centre
(c) its second focus
(d) the centre of curvature of its second surface
Answer: (c) its second focus
In simple words: Parallel rays always go to the second focus (in convex lens) or appear to come from second focus (in concave lens). This is the most important lens rule.

📝 Teacher's Note: Use sunlight and magnifying glass to show that parallel rays from sun meet at one point - the focus. This makes the concept very clear for students.

🎯 Exam Tip: For parallel ray questions, always choose "second focus". This is the standard lens rule that examiners expect you to know.

Exercise 5(B)

Question 1. What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer.
Answer: The three principal rays are:
(i) A ray of light incident at the optical centre O of the lens passes undeviated through the lens.
(ii) A ray of light incident parallel to the principal axis of the lens, after refraction passes through the second focus F₂ (in a convex lens) or appears to come from the second focus F₂ (in a concave lens).
(iii) A ray of light passing through the first focus F₁ (in a convex lens) or directed towards the first focus F₁ (in a concave lens), emerges parallel to the principal axis after refraction.
[Diagram: Shows three rays for both convex and concave lenses - ray through optical centre, parallel ray going to/from focus, and ray from focus becoming parallel.]
In simple words: These three rays are like drawing rules for lenses. Ray 1 goes straight through centre. Ray 2: parallel becomes focused. Ray 3: focused becomes parallel.

📝 Teacher's Note: Make students practice drawing these three rays many times. Tell them these are like magic rules - they work for every lens problem. Use different colored pens for each ray.

🎯 Exam Tip: Always draw all three rays neatly with arrows. Label the rays clearly. Remember: centre ray is straight, parallel ray goes to focus, focus ray becomes parallel.

Question 2. In the diagram below, XX' represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of rays A and B as they emerge out of the lens.
Answer:
[Diagram: Shows completion of ray paths A and B through convex and concave lenses according to the three principal ray rules.]
The ray paths are completed using the principal ray rules. Ray A (parallel to axis) goes through/appears from focus F. Ray B follows the appropriate lens rule based on its path.
In simple words: We use the three lens rules to draw where each ray goes after passing through the lens. Each ray follows its own rule.

📝 Teacher's Note: Give students many such diagrams to complete. Make them identify which rule applies to each ray first, then draw the path. Practice makes this automatic.

🎯 Exam Tip: First identify which type of ray it is, then apply the correct rule. Draw neat lines with arrows. Label important points like focus and optical centre.

Question 3. Distinguish between a real and a virtual image.
Answer:

In simple words: Real images can be caught on a screen and are upside down. Virtual images cannot be caught on a screen and are right way up. Think of a movie screen (real image) versus your reflection in a mirror (virtual image).

📝 Teacher's Note: Show students how a projector makes a real image on the screen. Then show them their reflection in a mirror - that's virtual. This makes the difference very clear.

🎯 Exam Tip: Always mention "can be obtained on screen" for real and "cannot be obtained on screen" for virtual. Also write "inverted" and "erect" clearly.

Question 4. Study the diagram (Fig. 5.55) given below.
(a) Name the lens LL'
(b) What are the points O and O' called?
(c) Complete the diagram to form the image of the object AB.
(d) State the three characteristics of the image.
(e) Name a device in which this action of lens is used.
Answer:
(a) LL' lens is convex lens.
(b) O and O' are known as first and second focal points respectively.
(c)

[Diagram: This diagram shows ray diagram construction for a convex lens with object placed between focus and lens, showing formation of magnified virtual image]

(d) The image formed will be magnified, virtual and upright.
(e) Such action of lens is used in a magnifying glass.
In simple words: A convex lens works like a magnifying glass when the object is very close to it. It makes things look bigger and right way up.

📝 Teacher's Note: Bring a magnifying glass to class. Let students look at their fingers through it. They will see exactly what this question describes - bigger, upright image.

🎯 Exam Tip: For convex lens used as magnifying glass, always write: magnified, virtual, upright. Remember the object must be between lens and focus.

Question 5. Study the diagram (Fig. 5.54) below.
(i) Name the lens LL'
(ii) What are the points O, O' called?
(iii) Complete the diagram to form the image of the object AB.
(iv) State three characteristics of the image.
Answer:
(i) LL' is concave lens.
(ii) The points O and O' are called second and first focal points respectively.
(iii)

[Diagram: This diagram shows ray diagram construction for a concave lens showing formation of diminished virtual image]

(iv) The three characteristics of the image are:
• Virtual
• Erect
• Diminished
In simple words: A concave lens always makes things look smaller and right way up. It spreads light rays apart instead of bringing them together.

📝 Teacher's Note: Show students the bottom of a glass bottle or safety glasses for short sight. These are concave and always make things smaller.

🎯 Exam Tip: For concave lens, the image is ALWAYS virtual, erect, and diminished. No matter where you place the object. This never changes.

Question 6. Study the diagram shown in Fig. 5.56
(a) Complete the diagram to show the formation of image A'B' of the object AB of same size.
(b) Name the lens LL' and draw its outline.
(c) What are the points O and O' called?
(d) Where is the object located?
(e) Where is the image formed?
(f) What are the two other characteristics of the image?
Answer:
(a) Complete diagram for the formation of A'B'.
(b) LL' is a convex lens which is drawn as

[Diagram: Drawing shows the outline shape of a convex lens - thick in middle, thin at edges]

(c) O and O' are called as first and second focal points respectively.
(d) Object is located at a distance twice of first focal length.
(e) Image is formed at a distance twice of second focal length.
(f) The image formed is real and inverted.
In simple words: When object is at 2F, the image forms at 2F on the other side. Both are same size but the image is upside down and real.

📝 Teacher's Note: This is a special case - object and image are same size. Draw two points at 2F on both sides of lens to show students this symmetry.

🎯 Exam Tip: Write "object at 2F₁, image at 2F₂, same size, real and inverted." This is the only position where object and image are equal size.

Question 7. The following diagram in Fig. 5.57 shows an object AB and a converging lens L with foci F₁ and F₂.
(a) Draw two rays from the object and complete the diagram to locate the position of the image. Mark the image CD. Clearly mark on the diagram the position of the eye from where the image can be viewed.
(b) State three characteristics of the image in relation to the object.
Answer:
(a) The complete diagram is

[Diagram: This diagram shows ray diagram with object between focus and lens, forming virtual magnified image on same side as object, with eye position marked]

(b) The image formed will be magnified, virtual and upright.
In simple words: When object is between lens and focus, we get a virtual image that is bigger and right way up. This is how reading glasses work.

📝 Teacher's Note: Explain that the eye must be on the right side of lens to see the virtual image. Draw dotted lines to show where light appears to come from.

🎯 Exam Tip: Always mark the eye position when asked. Virtual images can only be seen by placing eye on the opposite side of lens from object.

Question 8. The diagram given below in Fig. 5.58 shows the position of an object OA in relation to a converging lens whose foci are at F₁ and F₂.
(i) Draw two rays to locate the position of the image.
(ii) State the position of image with reference to the lens.
(iii) Describe three characteristics of the image.
(iv) Describe how the distance of the image from the lens and the size of the image change as the object is moved towards F₁.
Answer:
(i)

[Diagram: Ray diagram showing object beyond 2F₁ forming image between F₂ and 2F₂]

(ii) The position of the images will be more than twice the focal length of lens.
(iii) The image will be magnified, real and inverted.
(iv) As the object move towards F₁ the image will shift away from F₂ and it is magnified. At F₁ the image will form at infinity and it is highly magnified. Between F₁ and optical centre, the image will form on the same side of object and will be magnified.
In simple words: As you move the object closer to the lens, the image moves farther away and gets bigger. This is how a projector works.

📝 Teacher's Note: Use a torch and lens to show students how moving the object changes image position and size. This makes the concept very clear.

🎯 Exam Tip: Remember: object moves towards lens, image moves away from lens and gets bigger. Write "directly proportional" relationship between object distance and image size.

Question 9. A converging lens forms the image of an object placed in front of it beyond 2F₂ of the lens.
(a) Where is the object placed?
(b) Draw a ray diagram to show the formation of image.
(c) Where is the image formed?
(d) State three characteristics of the image.
Answer:
(a) The object is placed beyond 2F₁.
(b)

[Diagram: Ray diagram showing object beyond 2F₁ forming diminished real inverted image between F₂ and 2F₂]

(c) The image is formed beyond 2F₂.
(d) The image will be diminished, real and inverted.
In simple words: When object is very far from a convex lens, the image forms close to the lens and is smaller than the object. This is how cameras work.

📝 Teacher's Note: This is the camera case. Show students how distant objects form small images on camera film or sensor. Point to how mountains look tiny in photos.

🎯 Exam Tip: For object beyond 2F₁, image is always between F₂ and 2F₂. Write "diminished, real, inverted" - this never changes for this position.

Question 10. The following diagram in given below shows an object OA and its virtual image IB formed by a lens.
Answer:

[Diagram: Shows object and virtual image formation by a lens]

In simple words: This shows how a lens can form a virtual image. The image appears to be where the dotted lines meet, but no light actually goes there.

📝 Teacher's Note: Explain that virtual images are formed by extending the refracted rays backwards. The image appears to be there but is not really there.

🎯 Exam Tip: For virtual images, always draw dotted lines to show where the image appears to be. Solid lines show real light paths only.

Solution 10:
Answer:
(a) The lens is concave lens.
(b) The required diagram is

[Diagram: Shows a concave lens with object AB on the left, principal axis, and focal points F1 and F2 marked, with light rays showing divergence]

(c) The focal length is measured from optical centre C to F2.
In simple words: A concave lens spreads light rays apart. It is thinner in the middle and thicker at the edges. The focal length is the distance from the center to the focus point.

📝 Teacher's Note: Hold a magnifying glass backwards to show students how it makes things look smaller. That is how concave lens works - it spreads light apart instead of bringing it together.

🎯 Exam Tip: Always mention "diverging lens" for concave and write that focal length is measured from optical center to focus. Draw the lens correctly - thin in middle.

Question 11. The given below figure shows an object OA and its image IB formed by a lens.
(a) name the lens and show it in the diagram
(b) draw suitable rays to locate the lens and its focus.
(c) State three characteristics of the image.
Answer:
(a) Convex lens and it is shown in the diagram below.
(b)

[Diagram: Shows a convex lens with object and image positions marked, with ray diagram showing convergence of light rays]

(c) Three characteristics of the image are
(i) Magnified
(ii) Virtual
(iii) Upright
In simple words: The convex lens makes the image bigger, virtual (not on screen), and upright (same way up as object). This happens when object is very close to the lens.

📝 Teacher's Note: Use a magnifying glass to show students how words look bigger when lens is close. The image appears on same side as object - that makes it virtual.

🎯 Exam Tip: For magnifying glass setup, always write these three: magnified, virtual, upright. These are the key words examiners look for.

Question 12. A convex lens forms an image of an object equal to the size of the object.
(a) Where is the object placed in front of the lens?
(b) draw a diagram to illustrate it.
(c) state two more characteristics of the image.
Answer:
(a) The object is placed at the centre of curvature.
(b)

[Diagram: Shows object placed at center of curvature (2F1) with image formed at 2F2, both same size]

(c) The image formed is real and inverted.
In simple words: When object is at 2F (twice the focal length), the image is exactly same size. It forms on the other side and is upside down. You can catch it on a screen.

📝 Teacher's Note: Tell students this is the special position where object and image are same size. It is like a see-saw balanced in the middle.

🎯 Exam Tip: Write "object at 2F1" and "image at 2F2". Always mention real and inverted for this case. Same size means magnification = 1.

Question 13. A lens forms an erect, magnified and virtual image of an object.
(a) name the type of lens.
(b) Where is the object placed in relation to the lens?
(c) Draw a ray diagram to show the formation of image.
(d) name the device which uses this principle.
Answer:
(a) Convex lens
(b) The object is placed between the lens and focus (F1).
(c)

[Diagram: Shows object between F1 and lens, with diverging rays that appear to come from enlarged virtual image on same side]

(d) 'Magnifying glass' uses this principle.
In simple words: When you hold a magnifying glass very close to something, it looks bigger and stays upright. The image is not real - you cannot catch it on paper.

📝 Teacher's Note: Give each student a magnifying glass and let them look at their textbook. Move it closer and farther to see the effect. This hands-on experience helps them understand.

🎯 Exam Tip: For magnifying glass questions, always write "object between F and lens" and list all three: erect, magnified, virtual. These are must-have points.

Question 14. An object is placed on the axis of a lens. An image is formed by refraction in the lens. For all positions of the object on the axis of the lens, the positions of the image are always always between the lens and the object. (a) name the lens. (b) Draw a ray diagram to show it. (c) State three characteristics of the image.
Answer:
(a) Concave lens
(b)

[Diagram: Shows concave lens with object on one side and virtual image formed between lens and object, showing diverging rays]

(c) The image formed is virtual, upright and diminished.
In simple words: Concave lens always makes things look smaller and the image stays on the same side as the object. No matter where you put the object, this always happens.

📝 Teacher's Note: Show students the peephole in doors - that uses concave lens. Everything looks smaller through it. This helps students remember concave makes things smaller.

🎯 Exam Tip: Key clue is "image always between lens and object" - this only happens with concave lens. Write virtual, upright, diminished for characteristics.

Question 15. Classify as real or virtual, the image of a candle flame formed on a screen by a convex lens. Draw a ray diagram to illustrate how the image is formed.
Answer:
Let the candle is placed beyond 2F1 and its diminished image which is real and inverted is formed between F2 and 2F2.

[Diagram: Shows candle beyond 2F1 with real, inverted, diminished image formed between F2 and 2F2 on a screen]

Here the candle is AB and its real and inverted image is formed between F2 and 2F2.
In simple words: If you can see the image on a screen, it is real. The convex lens makes a real image when the candle is far away. The image is smaller and upside down.

📝 Teacher's Note: Use a candle and convex lens in a dark room. Students can see the flame image on the wall. This proves the image is real because it appears on screen.

🎯 Exam Tip: Key word is "screen" - only real images can form on screen. Write that object is beyond 2F and image is real, inverted, diminished.

Question 16. Show by a ray diagram that a diverging lens cannot form a real image of an object placed anywhere on its principal axis.
Answer:

[Diagram: Shows diverging lens with parallel rays coming from object that diverge after passing through lens, with virtual image formation shown by extending rays backward]

In simple words: Diverging lens spreads light rays apart. The rays never actually meet on the other side - they only appear to meet when you extend them backward. So image is always virtual.

📝 Teacher's Note: Draw diverging rays on board and show how they never actually cross on the other side. Only when extended backward do they appear to meet.

🎯 Exam Tip: Show rays diverging after the lens and use dotted lines to extend them backward to show virtual image formation. This proves no real image is possible.

Question 17. Draw a ray diagram to show how a converging lens can form a real and enlarged image of an object.
Answer:

[Diagram: Shows object placed between F1 and 2F1, with converging rays forming enlarged, real, inverted image beyond 2F2]

The image formed in above diagram is real, enlarged and inverted.
In simple words: When object is between F and 2F, the convex lens makes a bigger image on the other side. This image is real - you can catch it on screen.

📝 Teacher's Note: This is how movie projectors work. The film is between F and 2F, so we get big image on screen. Students can relate to this everyday example.

🎯 Exam Tip: Place object between F1 and 2F1 to get enlarged real image. Write that image is beyond 2F2 and is real, enlarged, inverted.

Question 18. Where will the image be formed if an object is kept in front of a concave lens at a distance equal to its focal length? Draw a ray diagram to illustrate your answer.
Answer:
The image will form between the focus and optical centre, on the same side of the lens as the object.

[Diagram: Shows object at focal length distance from concave lens, with virtual image formed between focus and optical center on same side]

In simple words: For concave lens, the image always forms on the same side as object. When object is at focus distance, image forms halfway between focus and lens center.

📝 Teacher's Note: Unlike convex lens where object at focus gives image at infinity, concave lens behaves differently. The image is always between object and lens.

🎯 Exam Tip: For concave lens, image is always virtual and on same side. Write position as "between focus and optical center" for this specific case.

Question 19. Draw a ray diagram to show how a converging lens is used as a magnifying glass to observe a small object. Mark on your diagram the foci of the lens and the position of the eye.
Answer:
The object is placed between focal point F1 and convex lens and its image is formed at the same side of the lens which is enlarged.
So this lens can be used as a magnifying lens.

[Diagram: Shows object between F1 and lens, with eye position marked, and enlarged virtual image formation shown with diverging rays]

In simple words: Put the object very close to magnifying glass (between focus and lens). Your eye sees a bigger, upright image. This is how reading glasses and magnifying glasses work.

📝 Teacher's Note: Let students use magnifying glass to read small print. Show them how moving closer or farther changes image size. Mark the best position.

🎯 Exam Tip: Mark F1, F2, object position, eye position clearly. Show virtual rays with dotted lines going back to eye. Image must be upright and enlarged.

Question 20. Draw a ray diagram to show how a converging lens can form an image of the sun. Hence give a reason for the term 'burning glass' for a converging lens used in this manner.
Answer:
The sun is at infinity so convex lens forms its image at second focal point which is real and very much diminished in size.

[Diagram: Shows parallel rays from sun (at infinity) converging to a point at F2, forming very small bright image]

In simple words: Sun rays are parallel. Convex lens brings all these rays to one tiny point at the focus. This tiny point gets very hot and can burn paper. That is why it is called burning glass.

📝 Teacher's Note: SAFETY WARNING: Never let students try this with real sunlight. Explain the concept but emphasize it can cause fires and eye damage. Use diagrams only.

🎯 Exam Tip: Draw parallel rays from left side converging at F2. Write that concentrated light energy at focus point creates heat - this explains "burning glass" term.

Question 21. A lens forms an inverted image of an object.
(a) what kind of lens is this?
(b) what is the nature of the image real or virtual?
Answer:
(a) This is convex lens.
(b) The nature of the image is real.
In simple words: When a lens makes an upside-down image, it is a convex lens. This upside-down image is real because light rays actually meet at that point.

📝 Teacher's Note: Show students a magnifying glass making an upside-down image on paper. This helps them see that convex lens makes inverted real images.

🎯 Exam Tip: Always write "convex lens" and "real image" together when the image is inverted. This is a key connection examiners look for.

Question 22. A lens forms an upright and magnified image of an object.
(a) name the lens
(b) State whether the image is real or virtual
Answer:
(a) Convex lens.
(b) Virtual.
In simple words: When the image is right-side up and bigger, it is made by a convex lens. This bigger upright image is virtual - you can see it but cannot catch it on paper.

📝 Teacher's Note: Use a magnifying glass to show students how it makes things look bigger and right-side up when held close. This is virtual image formation.

🎯 Exam Tip: Write "upright + magnified = virtual image by convex lens". Remember this simple rule for exams.

Question 23.
(a) name the lens which always forms an erect and virtual image.
(b) state whether the image in part (a) is magnified or diminished.
Answer:
(a) Concave lens
(b) Image is diminished
In simple words: Concave lens always makes things look smaller and right-side up. The image is always virtual - you can see it but cannot catch it on screen.

📝 Teacher's Note: Show students the lens in spectacles for short-sighted people. It makes everything look smaller. This is a concave lens.

🎯 Exam Tip: Write "concave lens always gives diminished virtual image". The word "always" is important for full marks.

Question 24. A lens forms an upright and diminished image of an object irrespective of its position. What kind of lens is this?
Answer: Concave lens
In simple words: No matter where you put the object, concave lens always makes it look smaller and right-side up. This happens because concave lens spreads out light rays.

📝 Teacher's Note: The key word is "irrespective of position" - this means no matter where the object is. Only concave lens does this.

🎯 Exam Tip: When question says "irrespective of position" or "at any distance", the answer is always concave lens.

Question 25. Give two characteristic of the image formed by a concave lens.
Answer: Image formed by a concave lens is virtual and diminished.
In simple words: Concave lens always makes two things happen - the image is smaller than the object and you cannot catch it on paper.

📝 Teacher's Note: Tell students to remember "concave = smaller + virtual". This simple rule covers all cases for concave lens.

🎯 Exam Tip: Always write both "virtual" and "diminished" for concave lens. Also add "upright" if question asks for three characteristics.

Question 26. Give two characteristic of the virtual image formed by a convex lens.
Answer: The virtual image formed by a convex lens will be magnified and upright.
In simple words: When convex lens makes a virtual image, it is always bigger than the object and right-side up. This happens when object is very close to the lens.

📝 Teacher's Note: Show students how a magnifying glass works when held very close to text. The text looks bigger and right-side up.

🎯 Exam Tip: For convex lens virtual image, always write "magnified and upright". These two words together give full marks.

Question 27. In each of the following cases, where must an object be placed in front of a convex lens so that the image formed is
(a) at infinity,
(b) of same size as the object,
(c) inverted and enlarged,
(d) upright and enlarged image?
Answer:
(a) at focus,
(b) at 2F,
(c) between F and 2F,
(d) between optical centre and focus.
In simple words: Different positions of object give different types of images. F means focus point, 2F means double the focus distance from lens.

📝 Teacher's Note: Draw a simple lens diagram on board showing F and 2F positions. Students should memorize these four positions and their image types.

🎯 Exam Tip: Write the positions exactly as given - "at focus", "at 2F", "between F and 2F", "between optical centre and focus". Use exact words.

Question 28. Complete the following table:

📝 Teacher's Note: Make students practice filling such tables. They should know that concave lens always gives virtual upright diminished images.

🎯 Exam Tip: For convex at focus, write "very much magnified". For concave at infinity, write "highly diminished". Use these exact size descriptions.

Question 29. State the changes in the position, size and nature of the image of an object when brought from infinity up to a convex lens. Illustrate your answer by drawing ray diagrams.
Answer:
(i) When the object is situated at infinity, the position of image is at F₂, it is very much diminished in size and it is real and inverted.
(ii) When the object (AB) is situated beyond 2F₁, the position of image (A'B') is between F₂ and 2F₂, it is diminished in size and real and inverted.
(iii) When the object (AB) is situated at 2F₁, the position of image (A'B') is at 2F₂, it is of same size as the object and real and inverted.
(iv) When the object (AB) is situated between 2F₁ and F₁, the position of image (A'B') is beyond 2F₂, it is magnified in size and real and inverted.
(v) When the object (AB) is situated at F₁, the position of image is at infinity; it is very much magnified in size and real and inverted.
(vi) When the object (AB) is situated between lens and F₁, the position of image (CD) is on the same side, behind the object; it is magnified in size and virtual and upright.

[Diagram: Multiple ray diagrams showing convex lens with object at different positions - at infinity, beyond 2F, at 2F, between F and 2F, at F, and between lens and F, with corresponding image formations]

In simple words: As object moves closer to convex lens, the image changes position and size. First it is small and real, then becomes big and real, finally becomes virtual when very close.

📝 Teacher's Note: Use a candle and convex lens to show all these cases practically. Students remember better when they see it happening.

🎯 Exam Tip: Always mention position, size and nature for each case. Write F₁, F₂, 2F₁, 2F₂ with subscripts. Draw neat labeled diagrams.

Question 30. State the changes in the position, size and nature of the image of an object when brought from infinity up to a concave lens. Illustrate your answer by drawing ray diagrams.
Answer:
(i) When object (AB) is situated at infinity then parallel rays from object appears to fall on concave lens. Due to which image forms at focus. This image is highly diminished in size and virtual and upright.
(ii) When object (AB) is situated at any point between infinity and optical centre of the lens then image forms between focus and optical centre. This image is diminished in size and virtual and upright.

[Diagram: Ray diagrams showing concave lens with object at infinity and at finite distance, showing virtual diminished upright images formed]

In simple words: For concave lens, no matter where you put the object, the image is always smaller, right-side up, and virtual. The image position changes slightly but nature stays same.

📝 Teacher's Note: Show students that concave lens behavior is much simpler than convex lens - it always gives the same type of image.

🎯 Exam Tip: For concave lens, always write "virtual, upright and diminished" for any object position. This is the key difference from convex lens.

Question 31. Complete the following sentences:
(a) An object is placed at a distance of more than 40 cm from a convex lens of focal length 20 cm. The image formed is real, inverted and ……….
(b) an object is placed at a distance 2f from a convex lens of focal length f. The size of image formed is ………….. that of the object.
(c) an object is placed at a distance 5 cm from a convex lens of focal length 10 cm. The image formed is virtual, upright and …………….
Answer:
(a) An object is placed at a distance of more than 40 cm from a convex lens of focal length 20 cm. The image formed is real, inverted and diminished.
(b) An object is placed at a distance 2f from a convex lens of focal length f. The image formed is equal to that of the object.
(c) An object is placed at a distance 5 cm from a convex lens of focal length 10 cm. The image formed is virtual, upright and magnified.
In simple words: (a) Object beyond 2F gives smaller real image (b) Object at 2F gives same size image (c) Object closer than F gives bigger virtual image.

📝 Teacher's Note: Help students recognize that 40 cm is more than 2F (2×20=40), 2f is exactly at 2F, and 5 cm is less than F (10 cm).

🎯 Exam Tip: First identify where the object is compared to F and 2F, then write the image properties. Use words "diminished", "equal", "magnified" exactly.

Question 32. State whether the following statements are 'true' or 'false' by writing T/F against them.
(a) A convex lens has a divergent action and a concave lens has a convergent action.
(b) A concave lens, if kept at a proper distance from an object, can form its real image.
Answer:
(a) False (F) - A convex lens has convergent action and a concave lens has divergent action.
(b) False (F) - A concave lens can never form a real image at any distance.
In simple words: (a) Convex lens brings light rays together (convergent). Concave lens spreads light rays apart (divergent). (b) Concave lens always makes virtual images only.

📝 Teacher's Note: Use torch and lens to show how convex lens focuses light to a point (convergent) and concave lens spreads light out (divergent).

🎯 Exam Tip: Remember: convex = convergent, concave = divergent. Also remember concave lens never makes real images. Write clear T or F.

Multiple Choice Type

Question 1. For an object placed at distance 20 cm in front of a convex lens, the image is at distance 20 cm behind the lens. The focal length of convex lens is:
(a) 20 cm
(b) 10 cm
(c) 15 cm
(d) 40 cm
Answer: (b) 10 cm
In simple words: When object and image are at same distance from lens, the object is at 2f distance. So 2f = 20 cm, which means f = 10 cm.

📝 Teacher's Note: Show students that when object distance equals image distance, the object is at 2f position. This is a special case that students should remember.

🎯 Exam Tip: Always write "2f = 20 cm, so f = 10 cm" to show your working. This gets you method marks even if calculation is wrong.

Question 2. For the object placed between optical centre and focus of a convex lens, the image is:
(a) real and enlarged
(b) real and diminished
(c) virtual and enlarged
(d) virtual and diminished
Answer: (c) virtual and enlarged
When the object is kept between optical centre and focus of a convex lens, the image is formed on the same side, behind the object. The image thus formed is virtual, enlarged and erect.
In simple words: When object is very close to convex lens (closer than focus), you get a bigger virtual image on same side. This is how magnifying glass works.

📝 Teacher's Note: Use a magnifying glass in class to show this. Hold it close to text and students will see enlarged letters on same side as object.

🎯 Exam Tip: Write "virtual, enlarged and erect" - all three properties together. Remember: between O and F gives virtual enlarged image.

Question 3. A concave lens forms the image of an object which is:
(a) virtual, inverted and diminished
(b) virtual, upright and diminished
(c) virtual, inverted and enlarged
(d) virtual, upright and enlarged
Answer: (b) virtual, upright and diminished
Concave lens forms virtual, upright and diminished image for all positions of the object.
In simple words: Concave lens always makes things look smaller and right-side up. No matter where you put the object, the image is always virtual, upright and smaller.

📝 Teacher's Note: Tell students that concave lens is like a "shrinking lens" - it always makes images smaller. Unlike convex lens which can make big or small images.

🎯 Exam Tip: Remember: concave lens ALWAYS gives virtual, upright, diminished image. No exceptions. Write all three properties.

Exercise 5(C)

Question 1. Define the term power of a lens. In what unit is it expressed?
Answer: The power of a lens is a measure of deviation produced by it in the path of rays refracted through it. Its unit is Dioptre (D).
In simple words: Power tells us how much a lens can bend light rays. Strong lenses bend light more. We measure it in Dioptre (D).

📝 Teacher's Note: Show students thick and thin lenses. Thick lens has more power - it bends light more. Like strong person can bend a stick more.

🎯 Exam Tip: Always write "deviation of light rays" and "unit is Dioptre (D)". These are key marking points.

Question 2. How is the power of a lens related to its focal length?
Answer: Power of lens (in D) = \( \frac{1}{\text{focal length (in metre)}} \)
In simple words: Power and focal length are opposite to each other. If focal length is small, power is big. If focal length is big, power is small.

📝 Teacher's Note: Use example: f = 0.5 m gives P = 2 D, f = 0.25 m gives P = 4 D. Students see that smaller f means bigger P.

🎯 Exam Tip: Write the formula P = 1/f and remember focal length must be in metres, not cm. Convert cm to m first.

Question 3. How does the power of a lens change if its focal length is doubled?
Answer: If focal length of a lens doubled then its power gets halved.
In simple words: If focal length becomes 2 times bigger, power becomes 2 times smaller. They work in opposite ways.

📝 Teacher's Note: Use numbers: if f = 0.2 m, P = 5 D. If f doubles to 0.4 m, P becomes 2.5 D (half of 5).

🎯 Exam Tip: Write "power gets halved" clearly. Show with P = 1/f that if f becomes 2f, then P becomes 1/2f = P/2.

Question 4. How is the sign (+ Or -) of power of a lens related to its divergent or convergent action?
Answer: The sign of power depends on the direction in which a light ray is deviated by the lens. The power could be positive or negative. If a lens deviates a ray towards its centre (converges), the power is positive and if it deviates the ray away from its centre (diverges), the power is negative.
In simple words: Convex lens brings rays together, so power is positive (+). Concave lens spreads rays apart, so power is negative (-).

📝 Teacher's Note: Use hands to show: bring fingers together for convex (+), spread fingers apart for concave (-). Students remember this action easily.

🎯 Exam Tip: Write "converges = positive power" and "diverges = negative power". Use the word "deviates" as given in definition.

Question 5. The power of a lens is negative. State whether it is convex or concave?
Answer: It is a concave.
In simple words: Negative power means the lens spreads light rays apart. Only concave lens does this.

📝 Teacher's Note: Remind students: negative = concave, positive = convex. Make them repeat this rule several times.

🎯 Exam Tip: Just write "concave lens" clearly. No need for long explanation in this type of question.

Question 6. What is magnifying glass? State its two uses.
Answer: Magnifying glass is a convex lens of short focal length. It is mounted in a lens holder for practical use. It is used to see and read the small letters and figures. It is used by watch makers to see the small parts and screws of the watch.
In simple words: Magnifying glass is a thick convex lens that makes small things look bigger. We use it to read small print or to see tiny parts clearly.

📝 Teacher's Note: Show a real magnifying glass. Let students use it to read small text. They will understand the two uses immediately.

🎯 Exam Tip: Write "convex lens of short focal length" for definition. Give two clear uses like reading small text and examining small objects.

Question 7. Draw a neat labelled ray diagram to show the formation of image by a magnifying glass. State three characteristics of the image.
Answer: Let the object (AB) is situated between focal length and optical centre of a convex lens then its image (A'B') will form on the same side of lens.

[Diagram: Shows a convex lens with object AB placed between optical centre and focus F. Light rays from the object pass through the lens and appear to come from virtual image A'B' which is enlarged and on the same side as the object.]

The image formed will be virtual, magnified and erect.
In simple words: When object is between lens and focus, we get a bigger virtual image on same side. The image is right-side up and bigger than object.

📝 Teacher's Note: Draw the ray diagram step by step on board. Show how two rays appear to meet on same side to form virtual image.

🎯 Exam Tip: Draw neat diagram with proper labels. Write three characteristics clearly: virtual, magnified, erect. These are standard marking points.

Question 8. Where is the object placed in reference to the principal focus of a magnifying glass, so as to see its enlarged image? Where is the image obtained?
Answer: The object is placed between the lens and principal focus. The image is obtained between the lens and principal focus.
In simple words: Put the object closer to lens than the focus point. The enlarged image appears on same side, also between lens and focus.

📝 Teacher's Note: Show with magnifying glass that object must be closer than focus to get enlarged image. Move object beyond focus and image disappears.

🎯 Exam Tip: Write "between lens and principal focus" for both object and image position. Use exact words from question.

Question 9. Define magnifying power of a simple microscope. How can it be increased?
Answer: The magnifying power of the microscope is defined as the ratio of the angle subtended by the image at the eye to the angle subtended by the object (assumed to be placed at the least distance of distinct vision D = 25 cm) at the eye.
Magnifying power = \( 1 + \frac{D}{F} \)
Where F is the focal length of the lens. The magnifying power of a microscope can be increased by using the lens of short focal length. But it cannot be increased indefinitely.
In simple words: Magnifying power tells us how many times bigger the image looks compared to seeing with naked eye. We can increase it by using lens with smaller focal length.

📝 Teacher's Note: Explain D = 25 cm is normal reading distance. Students should know this standard value for human eye.

🎯 Exam Tip: Write the formula clearly and state "use lens of short focal length" to increase magnifying power. Mention D = 25 cm.

Question 10. Describe in brief how would you determine the approximate focal length of a convex lens.
Answer: The approximate focal length of a convex lens can be determined by using the principle that a beam of parallel rays incident from a distant object is converged in the focal plane of the lens. In an open space, against a white wall, a metre scale is placed horizontally with its 0 cm end touching the wall. By moving the convex lens to and fro along the scale, focus a distant object on wall. The image which forms on the wall is very near to the focus of the lens and the distance of the lens from the image is read directly by the metre scale. This gives the approximate focal length of the lens.

[Diagram: Shows experimental setup with a metre scale placed against white wall, convex lens mounted on the scale, and distant object (tree) whose image is focused on the wall.]

In simple words: Point the lens at a distant tree or building. Move the lens until you get a clear image on white wall. Measure distance from lens to wall - that is the focal length.

📝 Teacher's Note: Do this experiment in class using sunlight and distant objects. Students love seeing the inverted image of trees on wall.

🎯 Exam Tip: Write the method step by step: setup scale, focus distant object, measure distance. Mention "parallel rays from distant object" in explanation.

Question 11. The diagram in Fig 5.68 shows the experimental set up for the determination of focal length of a lens using a plane mirror.
(i) draw two rays from the point O of the object to show the formation of image I at O itself.
(ii) What is the size of the image I?
(iii) State two more characteristics of the image I.
(iv) Name the distance of the objects O from the optical centre of the lens.
(v) To what point will the rays return if the mirror is moved away from the lens by a distance equal to the focal length of the lens?
Answer:
(i)

[Diagram: Shows object O at focal length from convex lens L, with plane mirror M behind the lens. Two rays from O pass through the lens, hit the mirror perpendicularly, and retrace their path to form image at O itself.]

(ii) The size of the image will be same as that of object.
(iii) The image formed will be real and inverted.
(iv) The distance of object O from optical lens will be equal to the focal length of the lens.
(v) The position of the mirror from lens does not affect the formation of image as long as the rays from the lens fall normally on the plane mirror M.
In simple words: When object is at focal length, rays become parallel after lens. Mirror sends them back and they meet at same point O. Image has same size as object.

📝 Teacher's Note: This is a clever method to find focal length. When object and image coincide at O, that distance is exactly the focal length.

🎯 Exam Tip: For part (iv), write "equal to focal length of lens". This is the key principle of this experiment. Draw rays parallel between lens and mirror.

Question 12. Describe how you would determine the focal length of a converging lens, using a plane mirror and one pin. Draw a ray diagram to illustrate your answer.
Answer: This is the same method as described in Question 11. Place a pin (object) in front of the lens. Place a plane mirror behind the lens. Adjust the distance of pin from lens until the pin and its image appear at the same position when viewed from the object side. At this position, the distance of pin from lens equals the focal length of the lens.

[Diagram: Shows the same setup as Question 11 with a pin as object O placed at focal length from lens L, with plane mirror M behind the lens.]

In simple words: Move the pin back and forth until its image appears exactly on top of the pin itself. At that point, the distance from pin to lens is the focal length.

📝 Teacher's Note: Demonstrate this with actual apparatus. Students are amazed when object and image coincide perfectly at focal length position.

🎯 Exam Tip: Draw the ray diagram clearly showing parallel rays between lens and mirror. Write "object distance = focal length when object and image coincide".

Question 12. To determine focal length by using plane mirror we need a vertical stand, a plane mirror, a lens and a pin.
Answer: To determine focal length by using plane mirror we need a vertical stand, a plane mirror, a lens and a pin. Place the lens L on a plane mirror MM' horizontally. Arrange a pin P on the clamp of a vertical stand such that the tip of pin is vertically above the centre O of the lens. Adjust the height of the pin until it has no parallax (i.e., when the pin and its image shift together) with its inverted image as seen from vertically above the pin. Now measure the distance x of the pin from the lens and the distance y of the pin from the mirror, using a metre scale and a plumb line. Calculate the average of the two distances. This gives the focal length of the lens, i.e., \( F = \frac{x + y}{2} \)
In simple words: We put a lens on a mirror. We move a pin up and down until we see it clearly with its upside-down reflection. Then we measure two distances and find their average. This gives us the focal length.

[Diagram: This diagram shows experimental setup with a lens placed on a plane mirror, a vertical stand with a pin, and light rays showing how the pin and its image align when there is no parallax.]

📝 Teacher's Note: Show students that parallax means things moving at different speeds when you move your head. When there is no parallax, the pin and image move together. This means they are at the same place optically.

🎯 Exam Tip: Always write the formula \( F = \frac{x + y}{2} \) clearly. Also mention "no parallax condition" - examiners look for this phrase.

Question 13. State two applications each of a convex lens and a concave lens.
Answer: The two applications of a convex lens are:

1. It is used as an objective lens in a telescope, camera, slide projector, etc.
2. With its short focal length it is also used as a magnifying glass.

The two applications of a concave lens are:

1. A person suffering from short sightedness or myopia wears spectacles having concave lens.
2. A concave lens is used as eye lens in a Galilean telescope to obtain an erect final image of the object.

In simple words: Convex lens makes things look bigger (magnifying glass) and captures light (camera). Concave lens helps short-sighted people see far things clearly and makes telescopes show upright images.

📝 Teacher's Note: Tell students to remember: convex lens converges (brings together) light rays, so it magnifies. Concave lens diverges (spreads apart) light rays, so it reduces image size.

🎯 Exam Tip: Write specific examples like "camera", "magnifying glass", "myopia spectacles". Generic answers like "optical instruments" get fewer marks.

Question 14. How will you differentiate between a convex and a concave lens by looking at (i) a distant object, (ii) a printed page?
Answer:
(i) On seeing a distant object through the lens, if its inverted image is seen, then the lens is convex, and if the upright image is seen, then the lens is concave.
(ii) On keeping the lens near a printed page, if the letters appear magnified, then the lens is convex, and if the letters appear diminished, then the lens is concave.
In simple words: Look through the lens at something far away. If it looks upside-down, it is convex. If it looks normal, it is concave. Hold it close to text - if letters look bigger, it is convex. If smaller, it is concave.

📝 Teacher's Note: Give students two lenses to try this experiment. They will remember better by doing it themselves. Remind them that distant objects and close objects behave differently with lenses.

🎯 Exam Tip: Write both tests clearly: "inverted image for convex" and "magnified letters for convex". Always mention what happens for both lens types.

Multiple Choice Type

Question 1. On reducing the focal length of a lens, its power:
(a) decreases
(b) increases
(c) does not change
(d) first increases then decreases
Answer: (b) increases
In simple words: Power and focal length are opposite to each other. When focal length gets smaller, power gets bigger. Think of it like this: a strong lens (high power) has short focal length.

📝 Teacher's Note: Show students the formula \( P = \frac{1}{f} \). When f gets smaller, P gets bigger. Use examples: thick spectacle lens (short focal length) has high power.

🎯 Exam Tip: Remember the formula \( P = \frac{1}{f} \). Write "Power is inversely proportional to focal length" to show you understand the relationship.

Question 2. A magnifying glass forms:
(a) a real and diminished image
(b) a real and magnified image
(c) a virtual and magnified image
(d) a virtual and diminished image
Answer: (c) a virtual and magnified image
In simple words: When you look through a magnifying glass, you see a bigger image that appears to be on the same side as the object. This image is virtual (cannot be caught on screen) and magnified (bigger).

📝 Teacher's Note: Let students use a magnifying glass to read small text. Show them that the image cannot be projected on a screen - that is what "virtual" means.

🎯 Exam Tip: For magnifying glass, always write "virtual and magnified". Also remember it forms upright image on same side as object.

Numericals

Question 1. The power of a lens is + 2.0 D. Find its focal length and state the kind of the lens.
Answer:
Given:
P = +2.0D

Step 1: Use power formula.
\( P = \frac{1}{f} \) (where f is in metres)

Step 2: Calculate focal length.
\( f = \frac{1}{P} = \frac{1}{2.0} = 0.5 \text{ m} = 50 \text{ cm} \)

Step 3: Determine type of lens.
Since power is positive (+2.0 D), it is a convex lens.

Answer: Focal length = 0.5 m or 50 cm, Convex lens
In simple words: We used the power formula to find focal length. Since power is positive, the lens is convex (converging lens).

📝 Teacher's Note: Teach students the sign convention: positive power means convex lens, negative power means concave lens. This is a very important rule.

🎯 Exam Tip: Always state the type of lens based on the sign of power. Write units clearly - focal length in metres for power calculation, then convert to cm.

Question 2. Express the power (with sign) of a concave lens of focal length 20 cm.
Answer:
Given:
Focal length = 20 cm = 0.2 m
Lens type = concave

Step 1: Use power formula.
\( P = \frac{1}{f} \) (where f is in metres)

Step 2: Calculate power.
\( P = \frac{1}{0.2} = 5 \text{ D} \)

Step 3: Apply sign convention.
As it is a concave lens, power is negative.
Therefore, P = -5 D

Answer: P = -5 D
In simple words: We calculated power using the formula. Since it is a concave lens, we put a negative sign in front of the answer.

📝 Teacher's Note: Emphasize that students must convert cm to metres before using the power formula. Also, concave lens always has negative power - this is sign convention.

🎯 Exam Tip: Always convert focal length to metres first. Write the negative sign clearly for concave lens power. The sign is very important for full marks.