NCERT Solutions For Class 10 Science physics chapter 9 Light Reflection and Refraction:
1. Which one of the following materials cannot be used to make a lens?
(a) Water. (b) Glass. (c) Plastic. (d) Clay
Solution : (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.
Solution :
(d) Between the pole of the mirror and its principal focus.
3. Where should an object be placed in front of 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.
Solution :
(b) At twice the focal length.
4. A spherical mirror and thin spherical lens have each of focal length of -15 cm. the mirror and lens are likely to be
(a) Both concave
(b) Both convex
(c) The mirror is concave and the lens is convex
(d) The mirror is convex and lens is concave.
Solution :
(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 concave or convex.
Solution :(d) either plane or convex
6. Which of the following lens 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
Solution :
(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 mirror? What is the nature of image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Solution :
"Object" must be placed in front of concave mirror between its pole and principal focus at a distance less than 15 cm. The image formed will be virtual and erect. The size of the image is larger the object. The ray diagram is as follows:
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.
Solution :
(a) Headlights of a car- concave mirror to give parallel beam of light after reflection from concave mirror.
(b) Side/rear-view mirror of vehicle- convex mirror as it forms virtual erect and diminished image to give wider view field.
(c) Solar furnace- concave mirror to concentrate sunlight to produce heat in solar furnace.
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 answers experimentally. Explain your observations.
Solution :
When one-half of a convex lens is covered with a black paper, this lens produces a complete image of the object. To prove it we perform experiment:
When another-half of a convex lens is covered with a black paper .
Take a concave mirror and cover half part of its by using black paper. Place it vertically in a stand. On one side of it place a burning candle. On opposite side of the lens fix a white screen. Adjust the position of candle or screen till clear image of burning candle is formed on the screen.
We observe that the image is complete image of the object. From the experimental observations, we find that image formation does not depend upon the size of a lens. A similar lens can also form complete image of an object placed in front of it. However, brightness of the image decreases when some part of lens is blocked. It is because now lesser number of rays pass through the lens.
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:
Given :
Height of object ho= 5 cm
Position of object u = - 25 cm
Focal length of lens f = 10 cm
Find : v= ? size hi = ? , and m = ?
Let us use the formula
1/v - 1/u = 1/f
1/v + 1/25 = 1/10
1/v = 1/10 - 1/25
1/v = (5 - 2)/ 50
1/v = 3/50
v = 50/3 = 16.67 cm
The distance of the image on the opposite side of the lens is 16.66 cm.
We know that
Magnification = v/u
m = 16.66/-25
m = -0.66
m = height of the image/ height of the object
-0.66 = height of the image/ 5 cm
Height of the image = -3.3 cm
A negative sign shows that an inverted image is formed , The position of the image is at 16.66 cm on the opposite side of the lens , The size of the image is -3.33 cm on the opposite side of the lens , The nature of the image will be real and inverted.
11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Solution :
f= -15 cm, v= -10 cm
1/v -1/u = 1/f
1/u = 1/15 – 1/10 = -1/30
u = -30 cm.
Ray diagram as follows:
12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of image.
Solution :
f = +15 cm, u = -10 cm.
1/f = 1/v +1/u
1/v = 1/15 +1/10
1/v = 5/30
v = + 30 cm.
The image is formed 6 cm behind the mirror, it is a virtual and erect image.
13. The magnification produced by a plane mirror is +1. What does this means?
Solution :