CBSE Class 10 Physics Light Reflection And Refraction Worksheet Set 04

This CBSE Class 10 Physics printable worksheet on Light – Reflection and Refraction (Chapter 9) has 24 short questions with answers and no multiple choice questions. It contains fill in the blanks, true or false statements and numericals on mirrors, lenses and power, all solved step by step for classroom or home practice.

Fill in the Blanks

Question 1. Fill in the blank(s): The power of a convex lens is ................. and that of a concave lens is ..............
Answer:
The power of a convex lens is positive and that of a concave lens is negative.
A convex lens has a positive focal length and a concave lens has a negative focal length, and P = 1/f.
In simple words: Convex lens: positive power; concave lens: negative power.

Teacher's Note:
1. The sign of the power follows the sign of the focal length.
2. Positive power means converging, negative means diverging.

Question 2. Fill in the blank(s): Light seems to travel in ...............
Answer:
Light seems to travel in straight lines.
This is why sharp shadows form and why we use the ray model of light.
In simple words: Light travels along straight lines in a uniform medium.

Teacher's Note:
1. Rectilinear propagation is the basis of ray diagrams.
2. It is observed with a beam of light passing through a hole.

Question 3. Fill in the blank(s): A light ray travelling obliquely from a denser medium to a rarer medium bends ............. the normal. A light ray bends .............. the normal when it travels obliquely from a rarer to a denser medium.
Answer:
Denser to rarer: bends away from the normal.
Rarer to denser: bends towards the normal.
In simple words: Light bends towards the normal in the denser medium and away from it in the rarer medium.

Teacher's Note:
1. The speed decreases in the denser medium.
2. The rule works for any pair of media.

Question 4. Fill in the blank(s): In case of a rectangular glass slab, the refraction takes place at both .................. interface and ............... interface. The emergent ray is ........... to the direction of incident ray.
Answer:
Refraction takes place at both the air–glass interface and the glass–air interface.
The emergent ray is parallel to the direction of the incident ray (with a lateral shift), because the two bends are equal and opposite.
In simple words: The ray bends in and then out by the same angle, so it leaves parallel to its original direction.

Teacher's Note:
1. The emergent ray is shifted sideways but not turned.
2. The shift increases with the thickness of the slab.

Question 5. Fill in the blank(s): Power of a lens is the reciprocal of its .................
Answer:
Power of a lens is the reciprocal of its focal length (measured in metres): P = 1/f.
In simple words: Power is one divided by the focal length in metres.

Teacher's Note:
1. Convert centimetres to metres first.
2. A short focal length gives a high power.

Question 6. Fill in the blank(s): The inner surface of the spoon can be approximated to a ........... mirror.
Answer:
The inner (curved-in) surface of a spoon can be approximated to a concave mirror, and the outer surface to a convex mirror.
In simple words: The hollow side of a spoon acts like a concave mirror.

Teacher's Note:
1. Hold the spoon close to your face to see the enlarged image.
2. The outer surface gives a small, erect image.

Question 7. Fill in the blank(s): The centre of the reflecting surface of a spherical mirror is a point called the ................
Answer:
The centre of the reflecting surface of a spherical mirror is called the pole (P).
In simple words: The middle of the mirror surface is its pole.

Teacher's Note:
1. The pole lies on the principal axis.
2. Distances are measured from the pole.

Question 8. Fill in the blank(s): The dentists use ............... mirrors to see large images of the teeth of patients.
Answer:
Dentists use concave mirrors to see large, erect images of the teeth, because the teeth are held between the pole and the focus.
In simple words: A concave mirror held close to the teeth gives an enlarged, upright image.

Teacher's Note:
1. Inside the focal length a concave mirror gives a virtual, enlarged image.
2. A convex mirror would give a smaller image.

Question 9. Fill in the blank(s): An object is placed in front of a spherical mirror. The image is found to be virtual for all positions of the object. The spherical mirror is .........
Answer:
The mirror is convex.
A concave mirror gives a real image when the object is beyond its focus, so only a convex mirror forms a virtual image for every position of the object.
In simple words: Only a convex mirror among spherical mirrors always gives a virtual image.

Teacher's Note:
1. Think about which spherical mirror cannot form a real image.
2. The image is also erect and diminished.

Question 10. Fill in the blank(s): Two immiscible transparent liquids A and B have 1.2 and 1.5 as their refractive indices (with respect to air). The refractive index of B with respect to A is ........
Answer:
n_BA = n_B/n_A = 1.5/1.2 = 1.25.
In simple words: Divide the refractive index of B by that of A to get 1.25.

Teacher's Note:
1. Both are measured with respect to air, so divide one by the other.
2. The answer is greater than 1 because B is denser than A.

True or False

Question 11. State whether true or false: The principal focus of a spherical mirror lies midway between the pole and centre of curvature.
Answer:
True. The focal length is half of the radius of curvature, f = R/2.
In simple words: The focus is halfway between P and C.

Teacher's Note:
1. f = R/2 is valid for mirrors of small aperture.
2. Check with R = 20 cm and f = 10 cm.

Question 12. State whether true or false: Convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
Answer:
True. A convex mirror is a diverging mirror and has a much wider field of view than a plane mirror, so it is used as a rear-view mirror.
In simple words: A convex mirror shows a larger area behind the vehicle.

Teacher's Note:
1. The image is smaller but the field of view is wider.
2. Mention both points in the answer.

Question 13. State whether true or false: A concave lens will always give a virtual, erect and diminished image.
Answer:
True. For every position of a real object, a concave lens forms an image between the lens and its focus on the same side as the object, which is virtual, erect and smaller.
In simple words: A concave lens always makes a small, upright, virtual image.

Teacher's Note:
1. Compare with a convex lens, which can also give real images.
2. The image is formed between O and F.

Question 14. State whether true or false: An object is placed in front of a mirror and an image of it is formed at the object itself. The mirror mentioned in question is a convex mirror.
Answer:
False. An image at the position of the object is formed when the object is at the centre of curvature of a concave mirror. A convex mirror always forms its image behind the mirror.
In simple words: It is a concave mirror, with the object at C.

Teacher's Note:
1. The image coincides with the object only at C of a concave mirror.
2. The image is real, inverted and of the same size.

Question 15. State whether true or false: Light travels faster in glass than in air.
Answer:
False. The speed of light in glass (about 2 × 10⁸ m/s) is less than that in air (about 3 × 10⁸ m/s), because glass is optically denser.
In simple words: Light is slower in glass.

Teacher's Note:
1. Glass has a refractive index of about 1.5.
2. v = c/n gives the speed in the medium.

Question 16. State whether true or false: The laws of reflection are valid for plane mirrors and not for spherical mirrors.
Answer:
False. The laws of reflection hold for every reflecting surface, plane or curved. For a curved mirror the normal at the point of incidence is the radius.
In simple words: The laws of reflection hold for all mirrors.

Teacher's Note:
1. For spherical mirrors the normal is the line joining the point of incidence to the centre of curvature.
2. Do not confuse this with mirror formula validity.

Question 17. State whether true or false: When a ray of light travels from air to water, its speeds up.
Answer:
False. Water is optically denser than air, so the speed of light decreases from about 3 × 10⁸ m/s to about 2.25 × 10⁸ m/s.
In simple words: Light slows down in water.

Teacher's Note:
1. Slower speed means the ray bends towards the normal.
2. Speed goes up only from denser to rarer medium.

Question 18. State whether true or false: A lens that is thicker at the middle than at the edges is a diverging lens.
Answer:
False. A lens thicker in the middle than at the edges is a convex (converging) lens. A concave (diverging) lens is thinner in the middle.
In simple words: Thick in the middle means a converging lens.

Teacher's Note:
1. The shape of a lens tells its type in air.
2. A convex lens in a denser medium can behave as a diverging lens.

Numericals

Question 19. A boy is standing in front of plane mirror finds his image at a distance of 20 m from himself. Find the distance between boy and mirror.
Answer:
In a plane mirror the image is as far behind the mirror as the object is in front.
If the boy is at distance d from the mirror, the distance between him and his image is 2d = 20 m.
So d = 10 m.
In simple words: Half of 20 m is the distance from the mirror, 10 m.

Teacher's Note:
1. Object distance equals image distance in a plane mirror.
2. The separation of object and image is twice the object distance.

Question 20. An object is placed at a distance of 12 cm in front of a concave mirror. It forms a real image four times larger than the object. Calculate the distance of the image from the mirror.
Answer:
The image is real, so m = −4 and u = −12 cm.
m = −v/u gives −4 = −v/(−12), so v = −48 cm.
The image is formed 48 cm in front of the mirror. (The focal length is f = −9.6 cm, as 1/f = −1/48 − 1/12.)
In simple words: Multiply the object distance by 4: the image is 48 cm from the mirror.

Teacher's Note:
1. For a real image the magnification is negative.
2. The same ratio v/u = 4 holds for the distances.

Question 21. Find the focal length of a lens of power –2.0 D. What type of lens is this?
Answer:
P = 1/f, so f = 1/P = 1/(−2.0) = −0.5 m = −50 cm.
The focal length (and power) is negative, so it is a concave (diverging) lens.
In simple words: f = −0.5 m, so it is a concave lens.

Teacher's Note:
1. The focal length comes out in metres when P is in dioptres.
2. Negative power means a concave lens.

Question 22. An object is placed 90 cm away from a concave mirror of focal length 30 cm. Find the position and the nature of the image formed.
Answer:
u = −90 cm, f = −30 cm.
\( \dfrac{1}{v}=\dfrac{1}{f}-\dfrac{1}{u}=-\dfrac{1}{30}+\dfrac{1}{90}=-\dfrac{2}{90} \), so v = −45 cm.
m = −v/u = −(−45)/(−90) = −0.5. The image is formed 45 cm in front of the mirror and is real, inverted and diminished (half the size).
In simple words: The image is 45 cm in front of the mirror, real, upside down and half the size.

Teacher's Note:
1. The object is beyond C (60 cm), so the image is between F and C.
2. m = −0.5 means inverted and half as tall.

Question 23. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.
Answer:
R = +30 cm, so f = +15 cm; u = −20 cm; h = 5 cm.
\( \dfrac{1}{v}=\dfrac{1}{15}+\dfrac{1}{20}=\dfrac{7}{60} \), so v = +8.57 cm (behind the mirror).
m = −v/u = +8.57/20 = +0.43, so h′ = 0.43 × 5 ≈ 2.14 cm.
The image is virtual, erect and diminished.
In simple words: The image is 8.57 cm behind the mirror, upright and 2.14 cm tall.

Teacher's Note:
1. For a convex mirror f is positive.
2. The image is always between P and F.

Question 24. Two thin lenses of focal lengths + 10 cm and – 5 cm are kept in contact. What is the focal length and power of the combination?
Answer:
P₁ = 1/0.10 = +10 D and P₂ = 1/(−0.05) = −20 D.
P = P₁ + P₂ = +10 − 20 = −10 D.
f = 1/P = 1/(−10) = −0.1 m = −10 cm. The combination behaves as a concave (diverging) lens.
In simple words: The powers add up to −10 D, so the combined focal length is −10 cm.

Teacher's Note:
1. For thin lenses in contact, powers add.
2. Use focal lengths in metres.