Two similar plano-convex lenses are combined together in three different ways as shown in the adjoining figure. The ratio of the focal lengths in three cases will be

Options

A. `2 : 2 : 1`
B. `1 : 1 : 1`
C. `1 : 2 : 2`
D. `2 : 1 : 1`

Answer

B

Explanation

(b) In each case, there is a combination of two plano-convex lenses placed close to each other. Focal length of combination `1/F=1/f+1/f` is same in all cases, so ratio of focal length in three cases is 1 : 1 : 1