Question

28.
The focal length of a concave lens is 30 cm. At what distance should the object
be placed from the lens so that it forms an image at 20 cm from the lens ?​

Answer 1

Explanation:

f= -30 cm

v= -20 cm

we know,

1/f= 1/v - 1/u

therefore.

1/-30 = 1/-20 - 1/u

-1/30 + 1/20 = -1/u

-20+30/600= -1/u

10/600= -1/u

1/60=-1/u

u= -60cm

therefore,

answer= -60 cm

Answer 2

Focal length =30cm
1/f=1/v-1/u
1/-30=-1/20-1/v
So v60 cm