Two thin equiconvex lenses each of focal length `0.2` m are placed coaxially with their optic centres `0.5` m apart.Then the foacla length of the combination is

Options

A. `-0.4` m
B. `0.4` m
C. `-0.1` m
D. `0.1` m

Answer

A

Explanation

`1/f = 1/(f_(1))+(1)/(f_(2)) - (d)/(f_(1)f_(2))`
or `1/f = 1/(0.2) + (1)/(0.2) - (0.5)/((0.2)(0.2))`
or `i/f = 5 + 5 - 0.5 xx 5 xx 5`
or `1/f = 10 - 12.5 = -2.5`
or `f = -(1)/(2.5) = -0.4 m`