In the given figure, the radius of curvature of curved surface for both the plano-convex and plano-concave lens is 10 cm and refractive index for both is `1.5`. The location of the final image after all the refractions through lenses is

Options

A. 15 cm
B. 20 cm
C. 25 cm
D. 40 cm

Answer

B

Explanation

The focal length of the plano-convex lens is
`1/f = (1.5-1) (1/(+10)- (1)/(oo)) = 1/20`
Focal length of plano-concave lens is
`1/f =(1.5-1) (1/oo - 1/10) = (-1)/(20)`
Since parallel beams are incident on the lens, its image from plano-concave lens will be formed at `+20` cm from it (at the forcus) and will act as an object for the plano-concave lens. Since the two lens are at a distance of `10 cm` from each other, therefore, cor the next lens `u= +10 cm`.
`:. v = (uf)/(u+f) = (10 xx 20)/(10 - 20) = 20 cm`