Question

A converging lens of focal length `50cm` is placed co-axially in cintact with another lens of unknown focal length. If the combination behaves like a diverging lens of focal length `50cm`, find the power and nature of second lens.

Answer 1

Here, `f_(1) = 50 cm`,
`P_(1) = (100)/(f_(1)) = (100)/(50) = 2 D`
`F = - 50 cm, P = (100)/(F) = (100)/(-50) = -2D`
`:. P_(1) + P_(2) = P`
`:. p_(2) = P - P_(1) = -2D - 2D = - 4D`
As power is negative, lens must be concave.