Question

A paralel beam of light incident on a concave lens of focal length 10 cm emerges as a parallel beam from a convex lens placed coaxially, the distance between the lenses being 10 cm. The focal length of the convex lens in cm is
A. 10
B. 15
C. 20
D. 30

Answer 1

Correct Answer - C

Image by concave lens is formed at its focus i.e., at `I_1`, after refraction from concave lens, rays become parallel to principal axis i.e., they are coming from focus of convex lens.
`f_2=10+10=20cm`