Two lenses of power `-15D` and `+5 D` are in contact with each other. The focal length of the combination is

Options

A. `-20 cm`
B. `-10 cm`
C. `+20 cm`
D. `+10 cm`

Answer

B

Explanation

`P_(1) = (1)/(f_(1)) = - 15 D , P_(2) = (1)/(f_(2)) = 5D`
When thin lenses are kept in contact with each other , the effective focal length of the combination will be
`(1)/(f_("elf")) = (1)/(f_(1)) + (1)/(f_(2)) = -15 + 5 =-10`
`implies f_("elf") = -(1)/(10) = -0.1 m = -10 cm`