Selina Concise Solutions for ICSE Class 8 Physics Chapter 5 Light Energy

• Light is a form of energy which produces in us the sensation of sight i.e. we can see objects only when light falls on them and then reflected into our eye.
• Velocity of light in air or in vacuum is 300,000 km per second. Or 3 × 10⁸ ms^-1
• As light passes into different mediums its speed changes and depends upon the density of medium i.e. it decreases with increase in density i.e. it is 2.25 × 10⁸ m/s in water and 2 × 1⁸ ms^-1 in glass as water is denser than air (mu = 1.33) and glass is still optically denser than water (mu = 1.5) i.e. slower in water and still slower in glass.
• Light travels in a straight line.
• As light travels from one transparent medium to other transparent medium and falls oblique at another medium, its path changes and this change in path is called REFRACTION OF LIGHT.
• When ray of light travels from RARER (less-denser) to DENSER medium, it bends TOWARD the normal AND when it travels from a DENSER to a RARER medium it bends away from NORMAL.
• ANGLE of INCIDENCE: “The angle which incident ray makes with normal”. “∠ i”
• ANGLE OF REFRACTION: “The angle which refracted ray makes with normal” “∠ r”. ∠ i is not equal to ∠ r.
• LAWS OF REFRACTION or SNELL’S LAWS OF REFRACTION:
  (i) Incident ray, normal at the point of incidence and Refracted ray all lie in the same plane.
  (ii) Ratio of sine of angle of incidence to the sine of angle of refraction is constant.
  frac{sin ∠ i}{sin ∠ r} = Constant = Refractive index = mu (mew)
  R.I. = mu = frac{Speed of light in air}{Speed of light in medium} = frac{Real depth}{Apparent depth}
• EFFECTS OF REFRACTION:
  (i) A coin placed in water appears to be raised.
  (ii) Swimming pool seen from above appears SHALOW.
  (iii) A pencil in water appears to be bent.
  (iv) MIRAGE in desert, EARLY Sunrise, LATE SUN set are all due to RFRACTION of light.
• White light is a band of seven colours-VIBGYOR. Speed of all colours of the white light in AIR or VACUUM is same, but different different transparent mediums.
• In glass or water Speed of VIOLET colour is MINIMUM and speed of RED light is MAXIMUM.
• Refractive index of medium is minimum for VIOLET light and R.I. of medium is maximum for red light.
• DISPERSION: “The splitting (breaking) of white light into seven colours is called DISPERSION OF LIGHT.
• CAUSE OF DISPERSION: Speed of different colours is different in glass or water and different colours get separated from each other on refraction at second surface of glass prism.

Test yourself

A. Objective Questions

1. Write true or false for each statement

Question (a): Water is optically denser than glass.
Answer: False. Water is optically denser than air.
Water has a refractive index of 1.33 while glass is higher at 1.5. Therefore, light travels slower in glass than in water, making glass the denser medium.
Teacher's Tip: Remember the values: Air (1.0), Water (1.33), Glass (1.5).
Exam Tip: Use the term "optical density" to describe how much a medium slows down light.

Question (b): A ray of light when passes from glass to air, bends towards the normal.
Answer: False.
When light moves from a denser medium like glass to a rarer medium like air, it speeds up. This increase in speed causes the ray to move away from the normal line.
Teacher's Tip: Use the mnemonic "DRA" - Denser to Rarer = Away.
Exam Tip: Always draw the normal perpendicular to the surface at the point of incidence.

Question (c): The speed of light is more in glass than in water.
Answer: False.
Glass is more optically dense than water, so it offers more resistance to light. Consequently, light travels faster through water (2.25 × 10⁸ m/s) than through glass (2 × 10⁸m/s).
Teacher's Tip: Denser materials act like "thicker traffic" for light, slowing it down.
Exam Tip: Memorize that speed is inversely proportional to refractive index.

Question (d): The depth of a pond when seen from above appears to be less.
Answer: True.
Refraction makes light rays coming from the bottom of the pond bend away from the normal as they enter the air. This causes our eyes to perceive the source of light at a shallower position than it actually is.
Teacher's Tip: This is called "Apparent Depth" and it is why clear pools look shallower than they are.
Exam Tip: In diagrams, use a dotted line to show the virtual position of the raised object.

Question (e): Light travels at a lower speed in water than in air.
Answer: True.
Air is considered an optically rarer medium compared to water. Light travels at its maximum speed in a vacuum or air but slows down upon entering the heavier medium of water.
Teacher's Tip: Think of light as a runner who slows down when they enter water.
Exam Tip: The speed of light in air is approximately 3 × 10⁸ m/s.

Question (f): Light travels in the same straight line path while passing through different media.
Answer: False.
The direction of light changes when it passes obliquely from one medium to another. This change in direction is what we call refraction.
Teacher's Tip: Light only travels in a straight line if it stays in the same medium.
Exam Tip: Mention "oblique incidence" because light falling normally does not bend.

Question (g): The angle formed between the normal and the refracted ray is known as the angle of incidence.
Answer: False.
The angle between the normal and the incident ray is the angle of incidence. The angle between the normal and the refracted ray is the angle of refraction.
Teacher's Tip: Incident ray = Angle of Incidence; Refracted ray = Angle of Refraction.
Exam Tip: Both angles are always measured from the normal, never from the surface.

Question (h): At the point of incidence, a line drawn at right angles to the surface, separating the two media, is called the normal.
Answer: True.
The normal is an imaginary reference line used to measure angles. It must always be drawn perpendicular to the interface of the two materials.
Teacher's Tip: "Normal" in math and physics simply means 90 degrees.
Exam Tip: Draw the normal as a dashed or dotted line to distinguish it from actual light rays.

Question (i): Image is formed by a mirror due to refraction of light.
Answer: False.
Mirrors work on the principle of reflection, where light bounces off a surface. Refraction is the bending of light as it passes through a material like a lens.
Teacher's Tip: Mirror = Bounce (Reflection); Lens = Pass through (Refraction).
Exam Tip: Do not mix up the terms 'Reflection' and 'Refraction' in mirror-related questions.

Question (j): Rays of light incident parallel to the principal axis pass through the focus after reflection from a concave mirror.
Answer: True.
A concave mirror is a converging mirror that brings parallel rays together. This common meeting point on the principal axis is designated as the Focus (F).
Teacher's Tip: Concave mirrors "cave in," bringing light to a center point.
Exam Tip: Use a ruler to ensure parallel lines stay parallel until they hit the mirror surface.

Question (k): A convex mirror is used as a shaving mirror.
Answer: False.
Shaving mirrors are concave mirrors because they can form an enlarged, erect image when held close to the face. Convex mirrors always form smaller (diminished) images.
Teacher's Tip: Shaving needs a "zoom," so we use concave mirrors.
Exam Tip: Identify the correct use of convex mirrors as 'rear-view mirrors' in vehicles.

Question (l): The focal length of a convex mirror is equal to its radius of curvature.
Answer: False.
The focal length (f) is exactly half of the radius of curvature (R). The formula relating them is R = 2f or f = R/2.
Teacher's Tip: The focus is the midpoint between the pole and the center of curvature.
Exam Tip: If the radius is 20 cm, the focal length must be 10 cm.

Question (m): A concave mirror converges the light-rays, but a convex mirror diverges them.
Answer: True.
Concave mirrors reflect rays inward toward a focus, making them useful for focusing energy. Convex mirrors reflect rays outward, appearing to come from a point behind the mirror.
Teacher's Tip: Converge = Come together; Diverge = Drive apart.
Exam Tip: Use arrows to show the direction of reflected rays clearly in your diagrams.

Question (n): A virtual image formed by a spherical mirror is always erect and situated behind the mirror.
Answer: True.
Virtual images are formed where light rays appear to meet but do not actually cross. Because they aren't real, they cannot be captured on a screen.
Teacher's Tip: Real images are upside down; virtual images are right-side up (erect).
Exam Tip: Always draw virtual images using dashed lines to show they aren't real.

2. Fill in the blanks

(a) Water is optically denser than air.
Air is the standard reference for a medium that light passes through easily. Water contains more molecules per unit volume, which slows down light more than air does.
Teacher's Tip: Rarer means thinner; Denser means thicker.
Exam Tip: Write "optically denser" rather than just "denser" for physics accuracy.

(b) Air is optically rarer than glass.
Light travels much faster in air because it is the least dense medium. Glass is a solid and slows light down significantly due to its high optical density.
Teacher's Tip: Speed of light in Air > speed of light in Glass.
Exam Tip: "Rarer" is the opposite of "Denser" in optics.

(c) When a ray of light travels from water to air, it bends away from the normal.
Moving from water to air means going from a denser to a rarer medium. This transition allows the light to speed up and deviate further from the normal line.
Teacher's Tip: Fast = Away from the Normal line.
Exam Tip: The angle of refraction (r) will be greater than the angle of incidence (i) here.

(d) When a ray of light travels from air to glass, it bends towards the normal.
As light enters the denser glass, it meets more resistance and slows down. This reduction in speed pulls the light ray closer to the imaginary normal line.
Teacher's Tip: Slow = Towards the Normal line.
Exam Tip: Ensure the angle of refraction is smaller than the angle of incidence in your sketch.

(e) When white light passes through a prism, it disperses.
A prism causes different wavelengths of light to bend by different amounts. This separation of colors happens at both the entering and exiting surfaces of the glass.
Teacher's Tip: Think of a prism as a "color separator."
Exam Tip: The white light must fall obliquely on the prism for dispersion to occur.

(f) The splitting of white light into its constituent colours is called dispersion.
White light is actually a mixture of seven primary colors. Dispersion reveals these hidden components by refracting them at different angles.
Teacher's Tip: Rainbows are a natural form of dispersion.
Exam Tip: Name the seven colors (VIBGYOR) if asked to explain dispersion in detail.

(g) A concave mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.
By silvering the outside, the inner curved surface becomes the reflecting side. This shape allows the mirror to collect and focus light rays inward.
Teacher's Tip: Outer silvered = Concave; Inner silvered = Convex.
Exam Tip: Use a diagram to show which side is shaded (silvered) and which is reflecting.

(h) Radius of curvature of a spherical mirror is two times its focal length.
The radius of curvature is the distance from the center of the original sphere to the mirror surface. The focus is located at the exact halfway point along this radius.
Teacher's Tip: R = 2f. Use this to check your math!
Exam Tip: Always state the units (like cm) when giving values for radius or focal length.

(i) The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is 0°.
A ray passing through the center of curvature hits the mirror surface at a 90-degree angle. In optics, falling "normally" means there is no angle relative to the normal itself.
Teacher's Tip: Normal = Center path. No angle = 0 degrees.
Exam Tip: These rays reflect back along their own path because ∠ i = ∠ r = 0.

(j) A convex mirror always forms a virtual image.
Convex mirrors always push light rays apart, so they never cross in real space. Our brain traces these diverging rays back to a point behind the mirror to create the image.
Teacher's Tip: Think of a convex mirror as the "always small and upright" mirror.
Exam Tip: Virtual images are always erect and diminished for convex mirrors.

(k) A concave mirror forms a virtual image for an object placed between pole and focus.
This is the only position where a concave mirror behaves like a magnifying glass. The rays diverge after reflecting, but their extensions meet behind the mirror.
Teacher's Tip: This is why concave mirrors are used for shaving or makeup.
Exam Tip: Remember this special case as it is the most frequently tested mirror position.

3. Match the following

Column A                              Column B
(a) A stone at a height          (i) power
(b) A moving ball                  (ii) joule
(c) Energy                            (iii) work done in 1 s
(d) Power                             (iv) potential energy
(e) watt                                 (v) kinetic energy

Answer:
(a) White light - (iv) spectrum
(b) Refraction - (v) ray of light from glass to air
(c) Virtual images - (i) convex mirror
(d) Real images - (ii) concave mirror
(e) Prism - (iii) refraction
These pairings link optical phenomena with the tools and materials that create them. For example, a prism uses refraction to create a spectrum from white light.
Teacher's Tip: Match the mirror types to the images they are most famous for.
Exam Tip: Read both columns fully before making your first connection.

4. Select the correct alternative

Question (a): The speed of light in air or vacuum is
Answer: 1. 3 × 10⁸ M s^-1
This is a fundamental physical constant representing the fastest speed anything in the universe can travel. It is slightly slower in air but we use this rounded value for most school calculations.
Teacher's Tip: That's 300,000 kilometers every single second!
Exam Tip: Pay close attention to the units; it must be m/s (meters per second).

Question (b): A ray of light moving from an optically rarer to a denser medium
Answer: 2. bends towards the normal
Moving from air to water or glass causes the light to meet more resistance. This slowing down forces the ray to take a path closer to the perpendicular normal line.
Teacher's Tip: Rarer to Denser = "Close to Normal."
Exam Tip: If the ray hits at 0 degrees (normal incidence), it does not bend at all.

Question (c): The angle between the normal and refracted ray is called
Answer: 3. angle of refraction
This is the formal name for the deviation light takes in the second medium. It is denoted by the symbol 'r' in most physics formulas.
Teacher's Tip: Angle of Incidence (i) is for the first medium; Angle of Refraction (r) is for the second.
Exam Tip: Always measure from the normal, not the glass/water surface.

Question (d): The property of splitting of white light into its seven constituent colours is known as
Answer: 4. dispersion
White light is composite, and dispersion is the act of unraveling it. This occurs because each color has a unique wavelength and speed in a medium.
Teacher's Tip: Dispersion creates the "VIBGYOR" rainbow.
Exam Tip: Don't confuse dispersion with refraction; refraction is the bending, dispersion is the splitting.

Question (e): The seven colours in the spectrum of sunlight in order, are represented as:
Answer: 1. VIBGYOR
This acronym stands for Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This sequence shows the colors in order of their increasing wavelength.
Teacher's Tip: Use the name "Roy G. Biv" backwards to remember the order.
Exam Tip: Violet always bends the most, and Red always bends the least.

Question (f): A ray of light passing through centre of curvature of a spherical mirror, after reflection
Answer: 4. retraces its own path.
Since the radius of a circle is always perpendicular to the surface, the light ray hits the mirror normally. According to the laws of reflection, it must bounce straight back.
Teacher's Tip: Think of this ray as hitting the mirror "head-on."
Exam Tip: This ray is very useful when drawing ray diagrams to find image positions.

Question (g): If the radius of curvature of a concave mirror is 20 cm, its focal length is:
Answer: 1. 10 cm
Focal length is defined as half of the radius of curvature (f = R/2). Dividing the 20 cm radius by two gives us the focal point at 10 cm.
Teacher's Tip: Radius is always the "big" number, Focal length is the "small" number.
Exam Tip: Always include the unit (cm) in your final answer.

Question (h): The image formed by a convex mirror is
Answer: 1. erect and diminished
Convex mirrors always provide a wider field of view by shrinking the images of objects. These images are always upright (erect) and located behind the mirror surface.
Teacher's Tip: "Diminished" just means made smaller.
Exam Tip: Convex mirrors never form real or enlarged images.

Question (i): The image formed by a concave mirror is of the same size as the object, if the object is placed
Answer: 4. at the centre of curvature.
At the Center of Curvature (C), the light rays cross exactly at the same distance from the mirror as the object. This results in an inverted image that matches the object's height perfectly.
Teacher's Tip: Position C is the "Mirror Match" position.
Exam Tip: The image at C is real, inverted, and same-size.

Question (j): A convex mirror is used
Answer: 3. as a rear view mirror by a driver
Drivers need to see as much of the road behind them as possible. Convex mirrors shrink the images of cars, allowing more of them to fit onto the small mirror surface.
Teacher's Tip: It says "Objects in mirror are closer than they appear" because the mirror makes them look small!
Exam Tip: Mention "wide field of view" as the reason for using convex mirrors in cars.

(B) Short/Long Answer Questions

Question 1: State the speed of light in (a) air, (b) water, and (c) glass
Answer: Speed of light in
Air - 3 × 10⁸ ms^-1 or 300,000 km/h
Water - 2.25 × 10⁸ ms^-1
Glass - 2 × 10⁸ ms^-1
These speeds show how light interacts with matter; the more atoms in the way, the slower it goes. Vacuum is the only place where light reaches its absolute top speed.
Teacher's Tip: As the material gets harder (Gas → Liquid → Solid), light gets slower.
Exam Tip: Use the standard 10⁸ scientific notation to show you understand physics units.

Question 2: How does the speed of light determine the optical density of a medium ?
Answer: If the speed of light in a medium is less than speed of light in air, this means the MEDIUM is DENSER than air. If speed of light is more than speed of light in AIR, this means the MEDIUM is LESS DENSER than air. i.e. speed of light is 1/density of medium
Optical density is a measure of how much a medium resists the flow of light. A high optical density means the medium is "stubborn" and forces light to crawl through it slowly.
Teacher's Tip: Inverse relationship: High speed = Low density; Low speed = High density.
Exam Tip: Use the term 'inversely proportional' to describe this relationship.

Question 3: Which is optically denser : water or air ? Give reason.
Answer: WATER is optically DENSER as speed of light is less in water. 2.25 × 10⁸ ms^-1 < 3 × 10⁸ms^-1 speed of light in Water < speed of light in Air.
Since light finds it harder to travel through water than air, water is considered the denser medium. This difference in density is what causes light to bend at the surface of a pool.
Teacher's Tip: Always compare the speeds to prove density.
Exam Tip: State both speeds (2.25 and 3) to make your answer more scientific.

Question 4: Out of air and glass, which is optically rarer ? Give reason.
Answer: Air is RARER speed of light in air is more than speed of light in glass. 3 × 10⁸ ms^-1 > 2 × 10⁸ m^-1 speed of light in air > speed of light in glass
Air has almost no particles compared to solid glass, allowing light to zoom through it at maximum speed. This lack of resistance makes it the rarer medium in this comparison.
Teacher's Tip: Rarer = Faster; Denser = Slower.
Exam Tip: Clearly indicate that speed is the 'reason' for the rarity.

Question 5: What do you understand by refraction of light ?
Answer: REFRACTION OF LIGHT: “The change in direction of path of light when it passes from one transparent medium to another is called REFRACTION OF LIGHT.”
This bending happens because light changes its speed the moment it hits a new material. It is the reason why objects underwater look distorted or shifted.
Teacher's Tip: Refraction = Bending at the border.
Exam Tip: Mention that both media must be 'transparent' for refraction to occur.

Question 6: Describe an experiment to show that a light ray bends when it passes from one transparent medium into another transparent medium.
Answer: EXPERIMENT : Spread and fix a sheet of whtie paper on the drawing board. At the centre of the paper, place a glass slab XYX ‘ Y’ and draw its boundiy. A ray of light AB travelling from air (rarer medium) to glass slab (denser medium). Part of path BC in denser medium bends towards the normal. ∠ r < ∠ i. This shows that when light travels from RARER to DENSER medium bends towards the normal. Ray BC travels from DENSER medium to RARER medium in air (RAY CD) bend away from normal. ∠ e > ∠ r. This shows that when a ray of light travels from DENSER to RARER medium bends AWAY from normal.
This glass slab experiment provides physical proof of light's bending behavior. By tracing the path with pins, we can see the ray shift its angle twice—once going in and once coming out.
Teacher's Tip: The entry bend and exit bend are in opposite directions!
Exam Tip: Use the terms 'Towards the normal' and 'Away from the normal' to score full marks.

Question 7: Draw a ray diagram to show that the depth of a vessel containing water when seen from above, appears to be less than its real depth.
Answer: When there is no water in the vessel (a) when water is added in the vessel coin appear to be at B than actually the coin is at A. Hence, the coin appear to be raised up in water. i.e. its height appears to be less than actually it is.
Light rays from the coin bend away from the normal as they exit the water. Since our brains assume light always travels in straight lines, we trace the rays back to a higher, virtual point.
Teacher's Tip: This is called "Apparent Upthrust" of light.
Exam Tip: In your diagram, draw one real ray and one virtual ray (dotted) to show the effect.

Question 8: Define the following terms : Incident ray, Refracted ray, Angle of incidence, Angle of refraction.
Answer: INCIDENT RAY AB : The ray light AB which is in air strikes the glass slab at B. Or “A ray of light falling on the surface separating the two media.”
REFRACTED RAY BK : A ray of light which after passing first medium is in second medium i.e. ray BK. “A ray of light travelling in other medium in the changed direction.”
ANGLE OF INCIDENCE : “The angle which the incident ray makes with the normal is called angle of incidence.” i.e. ∠ i
ANGLE OF REFRACTION : “The angle which the refracted ray makes with the normal is called angle of refraction.” i.e. ∠ r
These definitions form the basic vocabulary of optics. Understanding these parts allows us to mathematically predict how light will behave using Snell's Law.
Teacher's Tip: Everything is measured relative to the normal!
Exam Tip: When defining angles, always include the phrase "makes with the normal."

Question 9: A ray of light falls normally on a glass slab. What is the angle of incidence ?
Answer: When a ray of light falls on a glass slab normally, angle between normal and incident ray is zero. therefore Angle of incidence is zero.
Since the ray is traveling exactly along the normal line, there is no space between them to form an angle. Consequently, there is no bending, and the ray continues straight through.
Teacher's Tip: "Normally" means at 90 degrees to the surface, but 0 degrees to the normal.
Exam Tip: If ∠ i = 0, then ∠ r = 0. This is the only time light doesn't bend during refraction.

Question 10: A ray of light travels from a rarer medium to a denser medium. How will it bend ?
Answer: When a ray of light travels from a RARER medium to a DENSER medium, it bends TOWARDS the normal.
Entering a denser medium causes light to lose velocity. This "drag" pulls the light ray inward toward the center line.
Teacher's Tip: Rarer to Denser = Speed down = Towards Normal.
Exam Tip: Draw the refracted ray closer to the normal than the incident ray was.

Question 11: A ray of light travels from a denser medium to a rarer medium. How will it bend ?
Answer: When a ray of light travels from a DENSER medium to a RARER medium it will bend AWAY from normal.
As light "breaks free" into a rarer medium, it speeds up instantly. This sudden burst of speed pushes the ray outward, further from the normal.
Teacher's Tip: Denser to Rarer = Speed up = Away from Normal.
Exam Tip: In this case, the angle of refraction is always larger than the angle of incidence.

Question 12: The diagram given below in fig shows a ray of light AO falling on a surface separating two media. Draw the refracted ray in each, case.
Answer: Refracted ray O C is shown in each case. (a) Towards the normal or ∠ r < ∠ i. (b) Away from the normal or ∠ r > ∠ i. (c) Refracted ray goes undeviated or ∠ i = 0, ∠ r = 0
These cases illustrate the three fundamental behaviors of light refraction. Whether it bends in, out, or stays straight depends entirely on the densities and the angle of entry.
Teacher's Tip: Case (c) is "normal incidence" – it's the exception to the bending rule.
Exam Tip: Label each case (Towards, Away, Undeviated) for total clarity.

Question 13: Draw a diagram showing the refraction of a light ray from water to glass. Label on it the incident ray, the angle of incidence (i), and the angle of refraction (r).
Answer: Water is rarer than glass. Light travels from water to glass mean light travels from a rarer to a denser medium. The incident ray is AO. Angle of incidence is ∠ i. Angle of refraction is ∠ r.
Because glass is more optically dense (1.5) than water (1.33), light must bend towards the normal. The ray enters at angle i and continues at a smaller angle r through the glass slab.
Teacher's Tip: Even though both are transparent liquids/solids, one is still "rarer" than the other!
Exam Tip: Draw the ray bending inward to show you know glass is denser than water.

Question 14: The diagram in figure shows a ray of light AO falling on a rectangular glass slab PQRS. Complete the diagram till the ray of light emerges out of the slab. Label on the diagram the incident ray, the refracted ray and the emergent ray.
Answer: Incident ray AO. Refracted ray OB. Emergent ray BE are shown.
The light ray enters the glass and bends towards the normal (OB). When it leaves the glass back into the air, it bends away from the normal (BE) and ends up parallel to its original path.
Teacher's Tip: The entry ray and exit ray are always parallel in a rectangular slab.
Exam Tip: Mark the "Lateral Displacement" between the original path and the emergent ray.

Question 15: Explain the following : (a) A coin placed at the bottom of a vessel appears to be raised when water is poured in the vessel.
(b) A straight stick partly dipped in water obliquely, appears to be bent at the surface of water.
(c) The sun is seen before the sunrise and after the sunset.
Answer: (a) The coin at a appears to be at B i. e. depth of coin observed is less than actual depth at A. The ray of light starting from A (denser) medium bends away from the normal. Due to Refraction of light the coin appears at B at a lower depth. Hence in the same way the depth of water appear to be less deep.
(b) The phenomenon is due to REFRACTION OF LIGHT when light passing from denser (water) medium to (Air) rarer medium appears to come from Q’ Virtual image inspite of Q. Same is true for any other point of stick inside water. Stick appears to be raised or bent.
(c) EARLY SUNRISE and SUNSET: At sun Rise : When the Sun is just below the horizon, the light from the Sun suffers refraction from RARE to DENSER medium (As atmosphere is warmer than layers near the earth at that time) bends towards the normal at each refraction. Due to continuous bending of light rays, the Sun can be seen even when its actual position is just below the Horizon. As a result the Sun is seen in advance, two minutes before it rises above the horizon in the morning. Similarly, in the evening Sun is seen delayed by 2 minutes longer above the horizon after the Sun set.
Refraction affects how we see depth, shape, and even celestial bodies. Because air density changes with temperature and height, the entire atmosphere acts like a massive, curved lens for sunlight.
Teacher's Tip: Atmosphere = Big prism for the sun.
Exam Tip: Mention the "2-minute" time difference specifically for the sunrise/sunset question.

Question 16: What is mirage ? Give a reason for its formation ?
Answer: Hot sand (Rarer medium). MIRAGE : When it is very hot, an inverted image of tree is seen which is ‘illusion of eye’ (gives a false impression) of water under the tree. This is called a MIRAGE.” REASON: Sand becomes very hot during hot noon, the layers of air in contact become rarer (expand) while upper layers of air are still at comparatively low temperature and are denser medium. When rays of light from DENSER to RARER medium (starting from tree) are bent away from normal when refracted from Rarer to Denser medium (going towards eye) bend towards normal and a tree appears INVERTED.
A mirage is a trick played by hot air layers. Light from the sky bends upward toward our eyes, making us think we are seeing a reflection on water, when it is actually just refracted light from the sky.
Teacher's Tip: Hot air on the ground acts like a mirror.
Exam Tip: Mention 'Total Internal Reflection' for extra credit in higher-level questions.

Question 17: What is a prism ? Draw a ray diagram to show the refraction of a light ray through a prism.
Answer: PRISM “is a transparent refracting medium bounded by two plane surfaces inclined at some angle.” REFRACTING EDGE and BASE are shown in figure. QR is refracted ray. Emergent ray shown.
A prism has a unique shape that forces light to bend twice in the same general direction. This cumulative bending is what makes it so effective at dispersing white light into colors.
Teacher's Tip: Light in a prism always bends toward the base.
Exam Tip: Label the Angle of Prism (A) and the Angle of Deviation (D).

Question 18: What do you mean by the term dispersion ?
Answer: The splitting or breaking up of the white light into its constituent colours as it passes through a refracting medium such as prism is known as dispersion.
White light isn't a single thing, it's a team of seven colors. Dispersion is the process of having each team member run at their own pace, separating them for all to see.
Teacher's Tip: White in → Rainbow out = Dispersion.
Exam Tip: Clearly state that it happens through a 'refracting medium'.

Question 19: A ray of white light falls on a prism. Draw a ray diagram to show that the prism disperses the white light.
Answer: (Diagram shows white light entering and splitting into Red, Orange, Yellow, Green, Blue, Indigo, Violet on a screen).
Violet light travels the slowest in glass and therefore bends the most at each surface. Red light is the fastest and bends the least, staying at the top of the colored band.
Teacher's Tip: Violet is the "bottom" color in a prism diagram.
Exam Tip: Label the colors from top to bottom (Red to Violet).

Question 20: In figure AO is the ray of white light falling on a prism PQR. Complete the diagram till the light emerges out from the prism and falls on the screen.
Answer: Diagram is completed: ‘The white light splits up into seven colours which are seen on the screen and arrangement of colours from bottom to top is given by the word ‘VIBGYOR’.
The ray AO refracts inward and immediately begins to fan out into colors. Upon hitting the second surface, the fan spreads even wider before hitting the white screen.
Teacher's Tip: VIBGYOR is read from the bottom up in a prism diagram.
Exam Tip: Ensure your "screen" is drawn vertically and the light rays hit it in order.

Question 21: What do you understand by the term spectrum ? Name the various colours present in the spectrum of sunlight.
Answer: Spectrum is the band of colours obtained on a screen when white light passes through a prism and splits into its constituent colours. The colours of the spectrum are violet (V), indigo (I), blue (B), green (G), yellow (Y), orange (O), and red (R). The order of the spectrum is VIBGYOR.
The spectrum represents the full range of visible light our eyes can perceive. Each color has its own frequency and energy level, which gives it its unique appearance.
Teacher's Tip: Spectrum = The "Rainbow result" of dispersion.
Exam Tip: Memorize the VIBGYOR sequence; it is almost always part of the question.

Question 22: You are given a disc divided into seven sectors with colours violet, indigo, blue, green, yellow, orange and red in them. What would be its colour when it is rotated rapidly ?
Answer: Newton’s colour disc— It is a circular disc taken by Newton to demonstrate that the recombination of seven colours produce white light. A circular card board disc which is divided into seven sectors and they were painted with the seven colours of VIBGYOR. The disc when rotated at a very high speed appears WHITE.
Our eyes cannot distinguish the individual colors if they are moving too fast. Instead, our brain mixes all the overlapping color sensations into one, resulting in white.
Teacher's Tip: This proves that White = All Colors combined.
Exam Tip: Rapid rotation is the key; if it's slow, you still see individual colors.

Question 23: State the two laws of reflection of light.
Answer: (1) The incident ray, the reflected ray and the normal ray at the point of incidence, lie in the same plane.
(2) The angle of incidence i is equal to the angle of reflection r i.e. ∠ i = ∠ r
These laws govern how light bounces off surfaces like mirrors. They ensure that light behavior is predictable and consistent across all smooth, reflective materials.
Teacher's Tip: If light hits at 30 degrees, it bounces off at 30 degrees.
Exam Tip: Both laws must be stated together for a complete mark.

Question 24: What is a spherical mirror ?
Answer: Spherical mirror. “A mirror which is made from a part of a hollow sphere is called SPHERICAL MIRROR.”
Imagine taking a glass bowl and silvering either the inside or outside surface. These mirrors are much more powerful than flat ones because they can focus or spread light.
Teacher's Tip: It's like a slice from a giant glass orange.
Exam Tip: Mention that it is a "part of a hollow sphere."

Question 25: State the two kinds of spherical mirror and distinguish them with the aid of proper diagrams.
Answer: The kinds of SPHERICAL MIRRORS are : (i) CONCAVE MIRROR (ii) CONVEX MIRROR. IN CONCAVE MIRROR: Silvered surface is away from centre of curvature and focal length is negative, i.e. reflecting surface is towards centre of curvature. IN CONVEX MIRROR: Silvered surface is towards the centre of curvature and focal length is positive i.e. reflecting surface is away from the centre of curvature. It alway forms diminished (small) image which is VIRTUAL.
A concave mirror curves inward like a cave, while a convex mirror bulges outward like a hill. This difference in curvature completely changes how the mirror handles light rays.
Teacher's Tip: Concave = Inward curve; Convex = Outward bulge.
Exam Tip: In your diagram, use shading lines on the "silvered" side to show where the light *cannot* go.

Question 26: Explain the following terms : Pole, Centre of curvature, Radius of curvature, Principal axis. Show them on separate diagrams for each of the concave and convex mirrors.
Answer: POLE: “The geometric centre of spherical mirror is called POLE.” Or “The mid point of aperture AB of mirror is called pole.” P is pole of mirror.
CENTRE OF CURVATURE : “Is the centre of hollow sphere of which the mirror forms a part.” It is represented by the symbol ‘ C ‘.
RADIUS OF CURVATURE : “Is the radius of hollow sphere of which the mirror is a part”. It is represented by ‘R’.
PRINCIPAL AXIS: “Is a straight line joining the pole of the mirror to its centre of curvature and extended further.”
These terms help us map out the geometry of a mirror to calculate image positions. The Principal Axis is the imaginary "equator" that runs straight through the middle of the mirror's world.
Teacher's Tip: Pole is on the mirror; Center of Curvature is in front or behind it.
Exam Tip: Label all points (P, C, R) clearly on your diagram with a ruler.

Question 27: What do you understand by the focus and focal length of a spherical mirror ? Show them on the separate diagrams for each of a concave mirror and a convex mirror.
Answer: FOCUS : For concave mirror “Is a point on the principal axis at which the light rays incident parallel to the principal axis meet after reflection from the mirror.
FOCAL LENGTH: “The distance between focus and pole”. FP is focal length.
FOR CONVEX MIRROR: FOCUS: “of a convex mirror is a point on its principal axis at which the light rays incident parallel to the principal axis appear to meet after reflection from the mirror”.
FOCAL LENGTH: “The distance between P and F is F.L.”
The focus is where light "happens"—it's the point of convergence or divergence. The focal length tells us how "strongly" a mirror curves; a shorter length means a more dramatic curve.
Teacher's Tip: Focus (F) is a point; Focal length (f) is a measurement.
Exam Tip: For convex mirrors, use dotted lines to show the rays meeting at a "virtual" focus.

Question 28: Draw suitable diagrams to illustrate how a beam of light incident parallel to the principal axis is reflected by: (a) a concave mirror, and (b) a convex mirror
Answer: (a) Concave mirror: Parallel rays meet at point F in front of mirror.
(b) Convex mirror: Parallel rays bounce outward but extensions meet at point F behind the mirror.
These diagrams show why concave mirrors are called converging and convex mirrors are called diverging. One brings light to a focus while the other fans it out into the distance.
Teacher's Tip: Use a "meeting" diagram for concave and a "spreading" diagram for convex.
Exam Tip: Always include arrows pointing toward the mirror and away from it.

Question 29: How is a spherical mirror used to converge a beam of light at a point ? Name the type of mirror used.
Answer: If a parallel beam of light is incident on a concave mirror, it converges the beam to a point called focus. Type of mirror used is CONCAVE.
Because its surface curves inward, every ray hitting it is reflected toward a central axis. This makes it perfect for gathering solar energy or flashlight beams.
Teacher's Tip: Converge = "Bring together."
Exam Tip: Mention the 'Focus' as the point where the light gathers.

Question 30: How is a spherical mirror used to diverge a beam of light from a point ? Name the type of mirror used.
Answer: In case of convex mirror, the reflected rays diverge and do not meet at a point after reflection. They appear to come from a point’F on the principal axis. The type of mirror used is CONVEX mirror.
The outward curve pushes light rays apart, spreading them over a wide area. This is why these mirrors help drivers see a wide view of the traffic behind them.
Teacher's Tip: Diverge = "Split apart."
Exam Tip: Note that the rays only *appear* to come from the focus in a convex mirror.

Question 31: State the direction of incident ray which after reflection from a spherical mirror gets reflected along its own path. Give a reason.
Answer: A ray passing through THE CENTRE OF CURVATURE is incident normally on the spherical mirror, gets reflected back along its own path. A ray passing through the centre of curvature is reflected along its own path.
Since the ray comes from the center of the sphere, it hits the mirror's surface at a 90-degree angle. Just like a ball bouncing straight off a wall, it returns the exact same way it came.
Teacher's Tip: Normal rays are the "U-turn" rays of light.
Exam Tip: Give the reason as 'the ray falls normally on the mirror surface'.

Question 32: How is the focal length of a spherical mirror related to its radius
Answer: Focal length = Radius of curvature / 2 Or 2 F = R
This geometric relationship means the focus is always the midpoint between the mirror and its center point. It is a constant rule for all spherical mirrors, big or small.
Teacher's Tip: Radius is double, focal is half.
Exam Tip: Memorize R = 2f for quick calculation questions.

Question 33: The diagram (figure) given below shows two parallel rays 1 and 2 incident on (a) a concave mirror, (b) a convex mirror. Draw the reflected rays and mark the focus by the symbol F.
Answer: (a) A concave mirror: Rays 1 and 2 reflect and cross at F on the principal axis. (b) A convex mirror: Rays 1 and 2 bounce away but trace back to F behind the mirror.
This exercise tests your ability to apply the focal rules to multiple rays at once. The focus (F) is the landmark where all these parallel rays interact after bouncing.
Teacher's Tip: Always cross parallel rays at the focus (F).
Exam Tip: Use a ruler to make sure the crossing point is exactly on the Principal Axis.

Question 34: Complete the following diagrams in figure by drawing the reflected rays for the incident rays 1 and 2 if F is the focus and C is the centre of curvature.
Answer: For concave mirror: 1. Ray passing through F after reflection becomes parallel to principal axis. Ray (2) passing through centre of curvature travels back (retraces its path) i.e. reflected back along its own path. For convex mirror: Ray 1 becomes parallel to principal axis. Ray 2 replaces its path i.e. reflected back along its own path.
These are the two main "shortcut" rays used to find where an image will form. By tracing just these two, we can find where any object's image will appear.
Teacher's Tip: Ray through F goes Parallel; Ray through C bounces Back.
Exam Tip: These two rays are the easiest to draw for any image-finding question.

Question 35: Which are the two convenient rays that are chosen to construct the image by a spherical mirror for a given object ? Explain with the help of suitable ray diagrams.
Answer: Two convenient rays chosen to construct the image by a spherical mirror are: (i) A ray parallel to the principal axis which after reflection passes or appears to pass through focus. (ii) A ray passing through the centre of curvature or appears to pass through centre of curvature which retraces its path after reflection.
These two specific paths are easy to predict without complicated angle measurements. Where these two reflected paths cross is the exact location of the image.
Teacher's Tip: Parallel → Focus and Center → Center.
Exam Tip: Use a sharp pencil and ruler for all ray construction to ensure accuracy.

Question 36: Draw a ray diagram to show the formation of image of an object placed beyond the centre of curvature of a concave mirror. State the position, size and nature of the image.
Answer: OBJECT PLACED BEYOND C : Image is formed : A real, inverted and smaller image is formed between centre of curvature and focus. Position : between C and F. Size smaller than object.
As the object sits far away, the reflected rays converge quickly between the focus and center. The resulting image is a tiny, upside-down version of the original object.
Teacher's Tip: Think of this as the "miniaturized" image position.
Exam Tip: Mention all three properties: Position (C-F), Size (smaller), and Nature (Real/Inverted).

Question 37: Draw a ray diagram to show the formation of image of an object placed at the centre of curvature of a concave mirror. State the position, size and nature of the image.
Answer: Object placed at C : A real, inverted image of the same size is formed at the centre of curvature. Image formed A’B’ is at C the centre of curvature. Size is equal to the size of object AB. It is inverted and real.
This is a perfectly symmetrical setup where light travels equal distances to and from the mirror. It is the only position where the image is the exact same size as the object.
Teacher's Tip: At C = same size at C.
Exam Tip: This is a favorite exam diagram because it requires precise drawing to get the sizes equal.

Question 38: Draw a ray diagram to show the formation of image of an object placed between the focus and centre of curvature of a concave mirror. State the position, size and nature of the image.
Answer: OBJECT PLACED BETWEEN C and F : A real, inverted and magnified image is formed beyond the centre of curvature. The image formed A’B’ is beyond C ← position. Enlarged ← size. inverted and real ← nature.
As the object moves closer to the mirror, the reflected rays take longer to cross, making the image larger and further away. It's the opposite of the "Beyond C" case.
Teacher's Tip: Close object = Far, Large image.
Exam Tip: Use "Magnified" or "Enlarged" interchangeably to describe the size.

Question 39: Draw a ray diagram to show the formation of image of an object placed between the pole and focus of a concave mirror. State the position, size and nature of the image.
Answer: OBJECT LIES BETWEEN FOCUS AND POLE : A virtual erect and bigger image is formed behind the mirror. Image formed A’B’ is on the other side of mirror on producing ← position. Enlarged (magnified) ← size. Virtual and erect
← nature.
This is the "magnifying glass" effect of a concave mirror. Because the rays never cross in front, they form a large, upright ghost image that appears to be deep inside the mirror.
Teacher's Tip: This is the ONLY virtual case for a concave mirror.
Exam Tip: You MUST use dotted lines for the virtual rays behind the mirror.

Question 40: Draw a ray diagram to show the formation of image of an object placed on the principal axis of a convex mirror. State the position, size and nature of the image. What happens to the image as the object is moved away from the mirror ?
Answer: OBJECT PLACED IN FRONT OF CONVEX MIRROR : Place the object anywhere in front of convex mirror image formed is (i) Between pole and focus ← position. (ii) Diminished ← size. (iii) Upright (erect) and virtual
← nature. As the object is moved away from the mirror, the image shifts towards the focus and decreases in size but is (diminished).
Convex mirrors are simple because they always do the same thing: make everything small and upright. Moving away just makes the tiny image even tinier and closer to the focus point.
Teacher's Tip: Convex mirrors are the "wide-angle" mirrors of physics.
Exam Tip: Remember that convex mirrors NEVER produce real images.

Question 41: Draw separate diagrams for the formation of virtual image of an object by (a) concave mirror and (b) convex mirror. State the difference in the two images.
Answer: (a) See Q. No. 44, page 109. (b) See Q. No. 40, page 108.
(Enrichment provided below). The main difference is magnification: the concave mirror makes the virtual image much larger, while the convex mirror makes it much smaller. Concave virtual images are used for detail, while convex virtual images are used for a wider view.
Teacher's Tip: Concave zoom = BIG; Convex zoom = small.
Exam Tip: Draw both diagrams to show how rays meet behind the mirror in both cases.

Question 42: Name the mirror which always forms an erect and virtual image. What is the size of the image as compared to that of the object ?
Answer: It is CONVEX MIRROR. The size of image is smaller than the size of object, but when the object is at infinity the size of image is POINT size.
A convex mirror is designed to provide a "diminished" view. This helps squeeze many objects into a small mirror surface, which is essential for safe driving and security.
Teacher's Tip: Convex mirrors are the "shrinking" mirrors.
Exam Tip: Specify that it is 'smaller' than the object for full credit.

Question 43: Name the mirror which forms an erect, virtual and enlarged image of an object. What is the position of object relative to the mirror ?
Answer: Concave mirror forms an erect, virtual and enlarged image of an object. Since convex mirror always forms virtual, diminished, erect image. The position of object is between focus and pole of mirror as shown in figure ans. 39.
This magnifying effect is unique to the concave mirror when the object is very close. By placing your face between the mirror and its focal point, you see a zoomed-in, upright reflection.
Teacher's Tip: This is the "Beauty/Makeup Mirror" position.
Exam Tip: State the position as 'between Pole (P) and Focus (F)'.

Question 44: What is a real image ? Name the mirror which can be used to obtain the real image of an object. What should be the position of the object relative to the mirror ?
Answer: REAL IMAGE. “When rays of light after reflection or refraction actually meet at some other point” the image is real. Concave mirror. Except between pole and focus, the image formed is REAL.
Real images can be caught on a screen because light rays physically cross each other at that location. They are always inverted (upside down) compared to the original object.
Teacher's Tip: Real images are "Touch-able" with a screen.
Exam Tip: Mention that real images are formed in *front* of a mirror, not behind it.

Question 45: How can a concave mirror be used to obtain a virtual image of an object ? Draw a diagram to illustrate your answer.
Answer: A concave mirror forms a virtual image of an object, when object is placed between focus and mirror (pole). A virtual, erect and bigger image is formed behind the mirror.
In this close-up position, the light rays bounce off the mirror and spread out. Our brain traces these diverging paths back to a large crossing point behind the mirror's glass.
Teacher's Tip: Virtual rays = Dotted lines.
Exam Tip: Mark the image as 'A'B'' to distinguish it from the object 'AB'.

Question 46: State two uses of a concave mirror.
Answer: Two uses of concave mirror are : (1) Concave mirror is used as AREFLECTOR in head lights of cars and in search light. The source of light (bulb) is placed at the principal focus and the reflector forms parallel beam of light. (2) For doctors to examine throat, ear, nose and eyes, light is focused with the help of concave mirror.
Concave mirrors are excellent at manipulating light beams. They can either collect light into a tight, intense spot or push it out in a perfectly straight, powerful searchlight beam.
Teacher's Tip: Concave mirrors are "focuser" mirrors.
Exam Tip: Use the term 'converging' to describe why doctors like them.

Question 47: State two uses of a convex mirror.
Answer: Two USES OF CONVEX MIRROR: (i) It is used as A REAR VIEW MIRROR. (ii) It is used as a VIGILANCE MIRROR (iii) It is used as a REFLECTOR IN STREET LAMPS.
Convex mirrors provide a panoramic view of an area. They are often found in store corners or blind driveway turns to prevent accidents by showing a wide perspective.
Teacher's Tip: Convex mirrors are "safety" mirrors.
Exam Tip: Listing 'rear-view mirror' is the most essential point for this answer.

Question 48: A driver uses a convex mirror as a rear view mirror. Explain the reason with the help of a ray diagram.
Answer: A convex mirror always forms a small and upright image between pole and focus. That means in small area of mirror driver can see all the traffic coming from behind.
By creating a "diminished" image, the mirror fits a 180-degree view of the road onto a 10-centimeter piece of glass. This upright, mini-view helps drivers stay aware of everything happening around them.
Teacher's Tip: Wide field of view = Better traffic safety.
Exam Tip: Your ray diagram should show rays coming from two different objects at wide angles and fitting into the 'Eye' of the observer.

Question 49: State the kind of mirror used (a) by a dentist, and (b) as a street light reflector.
Answer: (a) A dentist uses CONCAVE MIRROR. (b) Convex mirror.
Dentists need a zoomed-in view of small teeth, which only a concave mirror can provide. Street lights use convex mirrors to spread the light over as much of the road as possible.
Teacher's Tip: Concave for small details; Convex for large areas.
Exam Tip: Specify the reason for each mirror choice (zoom vs spread).

Question 50: Name the kind of mirror used to obtain (a) a real and enlarged image (b) a virtual and enlarged image (c) a real and diminished image, and (d) a virtual and diminished image.
Answer: (a) CONCAVE (b) CONCAVE (c) CONCAVE (d) CONVEX.
Concave mirrors are versatile and can create almost any type of image depending on where you put the object. Convex mirrors are simpler and only have one "setting"—virtual and small.
Teacher's Tip: Concave is the "All-in-one" mirror; Convex is the "Mini-me" mirror.
Exam Tip: Memorize the image characteristic table for concave mirrors; it is the core of this chapter.