10 : Light – Reflection and Refraction

CBSE Class 10 Science Chapter 10 : Light – Reflection and Refraction

Chapter 10 . Light - Reflection and Refraction

Class 10 Science Chapter 10 . Light - Reflection and Refraction Intarnal Questions and Answers :

Intarnal Question :

1. Define the principal focus of a concave mirror.

Answer: A number of rays parallel to the principal axis are falling on a concave mirror and the rays are intersecting at a point on the principal axis of the mirror. This point is called the principal focus of the concave mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Solution: Here ,

We have,

Therefore, the focal length is 10 cm .

3. Name a mirror that can give an erect and enlarged image of an object.

Answer: concave mirror .

If the position of the object is between the pole and the focus , then the concave mirror gives an erect and enlarged image of an object .

4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Answer: Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards.

Example 10.1 : A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a bus is located at 5.00 m from this mirror, find the position, nature and size of the image.

Solution : Here, , and

Using mirror formula,

Therefore, the position of the object is 1.15 m .

Magnification

The nature of the of the image is virtual and erect .

The size of the image is diminished .

Example 10.2 : An object, 4.0 cm in size, is placed at 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image.

Solution: Here, , , , and

Using mirror formula,

Magnification

The screen should be placed at 37.5 cm from the mirror.

And

The height of the image is 6 cm .

The nature of the image is real and inverted .

The size of the image is enlarged.

Intarnal Question :

1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Solution: Here,

We have,

Therefore, the focal length of a convex mirror is 16 cm .

2. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located ?

Solution: For concave mirror :

Here, and

We have,

Therefore, the image is located within 30 cm .

Intarnal Question :

1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Answer: A ray of light travelling from the air ( a rarer medium ) to the water (a denser medium) slows down and bends towards the normal. When it travels from water (a denser medium) to the air (a rarer medium), it speeds up and bends away from the normal . So, the ray of light travelling in air enters obliquely into water .
2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is .

Solution: Here, The refractive index , and
We know that ,

Therefore, the speed of light in the glass is 2×108 ms-1 .
.3. Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

Answer: The medium with the highest optical density is diamond and the medium with lowest optical density is air .
4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.

Material medium	Refractive index
Kerosene	1.44
Turpentine	1.47
water	1.33

Answer : We have, Turpentine < Kerosene < water .

The light is traveling faster in the water .

5. The refractive index of diamond is 2.42. What is the meaning of this statement?

Answer: The refractive index of diamond is 2.42 . This means that the ratio of the speed of light in air and the speed of light in diamond is equal to 2.42 .

Example 10.3 : A concave lens has focal length of 15 cm. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also, find the magnification produced by the lens.

Solution: Here, , and

Using lens formula ,

The image distance is 6 cm .

Therefore, the magnification produced by the lens is 0.6 .

Example 10.4 : A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification.

Solution: Here, , , , and

Using lens formula , we have

Therefore, the image distance is 30 cm .

The magnification of the image is 2 . The image is two times enlarged.

And

The height of the image is 4 cm . The nature of the image is real and inverted.

Intarnal Question :

1. Define 1 dioptre of power of a lens.

Answer: 1 dioptre is the power of a lens whose focal length is 1 metre.

2. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Solution: Given, the image distance is equal to the size of the object distance ( i.e., ) .

Here, ,

Using lens formula,

Power of lens

Therefore, the power of the lens is – 4 D .
3. Find the power of a concave lens of focal length 2 m.

Solution: Here, (concave lens)

Therefore, the power of a concave lens is – 0.5 D .

Class 10 Science Chapter 10 . Light - Reflection and Refraction Exercise Questions and Answers :

1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay

Answer: (d) Clay
2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.

Answer: (d) Between the pole of the mirror and its principal focus.
3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer: (b) At twice the focal length .
4. A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be
(a) both concave.
(b) both convex.

Answer: (a) both concave.
5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) plane.
(b) concave.
(c) convex.
(d) either plane or convex.

Answer: (d) either plane or convex.
6. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.

Answer: (c) A convex lens of focal length 5 cm .
7. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.

Answer: For concave mirror ,

If the position of the object is located between the pole and the focus , then the position of the image behind the concave mirror . This means that the distance of the object from the concave mirror is between 0 to 15 cm .

The nature of the image is virtual and erect . The image is larger than the object .

8. Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason.

Answer: (a) Headlights of a car – concave mirror .

The concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light .
(b) Side/rear-view mirror of a vehicle – convex mirror .

Convex mirrors are preferred because they always give an erect, though diminished, image.
(c) Solar furnace – concave mirror .

The concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Answer: Covering one-half of a convex lens with black paper will not produce a complete image of the object. When this is verified experimentally, it is observed that only half of the object is visible, and the visible half is distorted in shape. The covered half of the lens remains dark, contributing no image information. This happens because the lens can only refract light from the uncovered half, resulting in an incomplete and distorted image.
10. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Solution: Here, ,