KENDRIYA VIDYALAYA, IIT CAMPUS, CHENNAI – 36

CLASS : X – SCIENCE

NUMERICAL TEST- 1

LN – 2 – LIGHT : REFLECTION AND REFRACTION:

1. A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall

object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.

2. An object 50 cm tall is placed on the principal axis of a convex lens. Its 20 cm tall

image is formed on the screen placed at a distance of 10 cm from the lens. Calculate the focal length of the lens.

3. An object 2 cm high is placed at a distance of 64 cm from a white screen. On placing a

convex lens at a distance of 32 cm from the object it is found that a distinct image of the object is formed on the screen. What is the focal length of the convex lens and size of the image formed on the screen? Draw a ray diagram to show the formation of the image in this position of the object with respect to the lens.

4. A convex lens has a focal length of 10 cm. At what distance from the lens should the

object be placed so that it forms a real and inverted image 20 cm away from the lens?

What would be the size of the image formed if the object is 2 cm high? With the help of a ray diagram show the formation of the image by the lens in this case.

5. The refractive index of glass is 1.50 and the speed of light in air is 3 x 108 ms-1.

Calculate the speed of light in glass.

6. At what distance should an object be placed from a convex lens of focal length 18 cm

to obtain an image at 24 cm from it on the other side. What will be the magnification produced in this case?

7. How far should an object be placed from a .convex lens of focal length 20 cm to

obtain its image at a distance of 30 cm from the lens? What will be the height of the image if the object is 6 cm tall?

8. The image of an object placed at 60 cm in front of a lens is obtained on a screen at a

distance of 120 cm from it. Find the focal length of the lens. What would be the height of the image if the object is 5 cm high?

9. (a) If the image formed by a lens is diminished in size and erect, for all positions of the

object, what type of lens is it? (b) Name the point on the lens through which a ray of light passes undeviated. (c) An object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find (i) the position (ii) the magnification and (iii) the nature of the image formed.

10.(а) Draw a ray diagram to show the formation of image of an object placed between

infinity and the optical centre of a concave lens. (b) A concave lens of focal length 15 cm forms an image 10 cm from the lens.

Calculate

(i) the distance of the object from the lens. (ii) the magnification for the image formed. (iii) the nature of the image formed.

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ANSWER KEY:

1.

2. Given : v = 10cm, h’ = -20 cm, h = 50cm

m = -20 / 50 = -2/5 (v/u)

m = -2/5 = v/u

m = -2/5 = 10 /u

u = -5 x 10 / 2 = -50/2 = - 25cm

u = - 25 cm

1/f = 1/v – 1/u

1/f = 1/ 10 – (- 1/25) = 5 + 2 / 50 = 7/50 = 7.14cm

3. Since, object-screen distance is double of object-lens separation, the object is at a

distance of 2f from the lens and the image should be of the same size of the object.

4.

5.

6.

7.

8.

9. (a) Concave lens.

(b) Optical centre. (c) u = – 30 cm, f = 20 cm

(iii) Image is real as v > 0, inverted and enlarged.

10. (a) Image formation when the object is at any position between infinity and optical

centre:

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