# Frank Solutions for Chapter 2.2 Refraction through a Lens Class 10 Physics ICSE

## Exercises

1. What do you mean by a lens?
A lens may be defined as a transparent refracting medium bounded by two curved surfaces which are generally spherical.

2. State two main differences between a convex and concave lens.
 Convex lens Concave lens 1. A convex lens is thicker in the middle and thinner at its edges.2. A light beam converges on passing through a convex lens. 1. A concave lens is thicker at the edges and thinner in the middle.2. A light beam diverges on passing through a concave lens.

3. Draw neat diagram to show the
(i) Convergent action of a convex lens,
(ii) Divergent action of concave lens.
(i) Diagram showing convergent action of convex lens :

4. Define the term focal length of a lens.
Focal length of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal lengths on either side of the lens and they are referred to as the first focal length and the second focal length of a lens:
1. First focal length: The distance from the optical centre of the lens to its first focal point is called the first focal length (f1) of the lens.
2. Second focal length: The distance from the optical centre of the lens to its second focal point is called the second focal length (f2) of the lens.

5. Can you guess the focal length of a plane mirror?
Focal length of a plane mirror is infinity.

6. Write the SI unit of focal length of a concave lens.
The SI unit of focal length is 'metre'.

7. Define the term principle axis of a lens.
Principal axis of a lens is the line joining the centres of curvatures of the two surfaces of the lens.

8. What do you understand by focal plane of a lens?
Focal plane of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal planes on either side of the lens and they are referred to as the first focal plane and the second focal plane of a lens:
1. First focal plane: It is the plane passing through the first focal point and normal to the principal axis of the lens.
2. Second focal plane: It is the plane passing through the second focal point and normal to the principal axis of the lens.

9. Show by a diagram the refraction of two incident rays parallel to the principle axis passing through a convex lens by treating it as a combination of a glass block and two triangular glass prisms.

10. Show by a diagram, the refraction of two incident rays parallel to the principle axis passing through a concave lens by treating it as a combination of a glass block and two triangular glass prisms.

11. State the condition for each of the following:
(i) A lens has both its focal lengths equal,
(ii) A ray passes undeviated through the lens.
(i) If the medium on both sides of the lens is same, the first and second focal lengths are equal.
(ii) A ray of light passing through the optical centre of the lens passes undeviated.

12. Fig. 26 shows a lens as a combination of a glass block and two prisms. (i) Name the lens formed by the combination .
(ii) What is the line XX' called ?
(iii) Complete the ray diagram and show the path of the incident ray PQ after passing through the lens.
(iv) The final emergent ray will either meet XX' at a point or appear to come from a point on XX'. What is this point called ?
(i) Convex lens.
(ii) The line XX' is called the principal axis.
(iii)
13. Fig. 27 shows a lens as a combination of a glass block and two prisms.
(i) Name the lens formed by the combination.
(ii) What is the line XX' called ?
(iii) Complete the path of the incident ray PQ after passing through the lens.
(iv) The final emergent ray either meets XX' at a point or appears to come from a point on XX'. Label it as F. What is this point called ?
(i) Concave lens
(ii) The line XX' is called the principal axis.
(iii)

14. An object of height 4.0 cm is placed at a distance 24 cm in front of a convex lens of focal length 8 cm. Draw a ray diagram to find the position of image. State the characteristics of the image.
Choose a proper scale say, 4 cm = 1 cm. Mark the lens LPL' on the principal axis XX'. In front of the lens, mark the object OA with distance OP = 24 cm and height of object OA = 4 cm.
For the object OA kept at a distance 24 cm in front of the convex lens of focal length 8 cm, the construction of  the image is shown in the figure. The distance PI of image IB from the lens is 12 cm. Thus, the distance of image is 12 cm. The image is real, inverted and diminished (size 2.0 cm).

15. Fig. 28 shows an object PQ placed on the principle axis of a lens L. The two foci of the lens are F1 and F2. The image formed by the lens is erect, virtual and diminished. (i) Draw the outline of the lens L used and name it.
(ii) Draw a ray of light starting from Q and passing through O.
Show the same ray after refraction by the lens.
(iii) Draw another ray from Q which is incident parallel to the principle axis and show how it emerges after refraction from the lens.
(iv) Locate the final image formed.

16. Distinguish between a real and a virtual image.
 Real image Virtual image 1. When the rays of light diverging from a point after reflection or refraction actually converge at some point, then that point is the real image of object.2. A real image is always inverted and can be taken on the screen.3. A real image is formed by eye, photographic camera, convex lens except when object is very close to the lens, concave mirror when the object is very close to concave mirro. 1. When the rays of light diverging from a point after reflection or refraction appear to diverge form some other point then the image is called virtual image.2. A virtual image is always erect and cannot be taken on the screen.3. A virtual image is formed by plane mirror, convex mirror and concave lens.

17. Refer to fig. 29 :
(i) Name the lens L.
(ii) What are the points O, O' called ?
(iii) Complete the diagram to form the image of the object AB.
(iv) Write three characteristics of the image.
(i) Concave lens
(ii) points O and O' are second focus and first focus respectively.
(iii)

18. A convex lens produces an image on a screen twice the size of the object. The distance between the object and screen is 45 cm. Find the distance of the lens from the object and its focal length by drawing a ray diagram. Magnification , m = I/O = u/v = 2 (given)
⇒ v = 2u
u + v = 45 cm  (given)
⇒ u + 2u = 45 cm
⇒ 3u = 45 cm
∴ u = 15 cm
Distance of lens from the object is 15 cm.
⇒ v = 2u = 30 cm

19. (i) Express power of a concave lens of focal length 50 cm with its sign.
(ii) The power of a lens is +2.0 D. Find its focal length and state what kind of lens it is.