Two thin lenses of focal lengths + 10 cm and - 5 cm are kept in contact. What is the focal length and power of the combination.

Explanation

Here , `f_1 = + 10 cm , f_2 = - 5 cm`
`therefore` Focal length of the combination of lenses is given by
`1/f = 1/f_1 + 1/f_2`
`implies 1/f = 1/(10) + 1/(-5) =1/(10) - 1/(5)`
`implies 1/f =(1-2)/(10)`
`implies 1/f = - 1/(10)`
`therefore f = - 10 cm`
Power of combination
`P = (1)/(f("in cm")) = (100)/(f"(in cm)")`
`=- (100)/(10) D`
`=-10 D`