## (b) Sunita's ophthalmologist suggests her to use a lens of power -2 D to correct her vision.

## (i) What type of lens should she use?

## (ii) What should be the focal length of the lens?

## (iii) An object is kept at 10 cm in front of the lens of power -2 D. Find the distance where the image is produced.

**
Answer:
**

(b)

(i)

-2D power =
**
- ½ metres
**
focal length

**
Concave lenses
**
have
**
negative focal lengths
**
.

Therefore, she should use a
**
diverging lens/ concave lens
**
.

(ii)

f = 1/P = 1/(-2) =
**
- 0.5 m
**

(iii)

**
Given
**
:

u = -10 cm

P=-2 D

f = -0.5m = -50 cm

1/f = 1/v - 1/u

v = uf/u+f

v = (-10)(-50)/-10 -50

v = -8.33 cm

Therefore, the image is produced
**
8.33cm in front of the lens
**
.