Question

An object is placed 30 cm from a convex lens .A real image is formed 20 cm from the lens .find the focal length.

Answer 1

Lens formula:1/f=1/v-1/u
u=30cm,v=20cm then focal length is
1/f=1/20-1/30
=30-20/60=10/60=1/6
1/f=1/6
f=6cm

Answer 2

Answer:Answer is 12.

Explanation:

Lens formula:-1/f=1/v-1/u

1/f=1/v-1/u

1/f=1/20-1/(-30)

1/f=1/20+1/30

1/f=30+20/600

1/f=50/600

1/f=1/12

f=12