Question

A point object is placed at a distance of 15 cm from a convex lens. The image is formed on the other side at a distance of 30 cm from the lens. When a concave lens is placed in contact with the convex lens, the image shifts away further by 30 cm. Calculate the focal lengths of the two lense

Answer 1

given :

object distance =u=15cm

Image distance =v=30cm

Let f1 be the focal length of convex lens .

From lens formula :

1/f=1/v-1/u

=1/30+1/15

=1+2/30=3/30

f1=30/3=10cm

As per the question, on placing concave lens image is shifted to 30cms.

So final image is =vf=30+30=60cm

Object distance from concave lens=u=30cm

fc= focal length=?

1/fc=1/v-1/u

=-1/60+1/30

= -2+1/60= -1/60

fc= -60cm

Hence focal length of convex lens is 10cm and concave lens is -60cm

Answer 2

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