Find the focal length and nature of lens which should be placed in contact with a lens of focal length `10 cm` so that the power of the combination becomes 5 dioptre.

Answer

`- 20 cm`, concave

Explanation

Here, `f_(1) = ?, f_(2) = 10 cm, P_(2) = 100/(f_(2)) = 100/10 = 10 D, P = 5 D`.
As `P_(1) + P_(2) = P, P_(1) + 10 = 5` or `P_(1) = = 5- 10 = - 5 D`. and `f_(1) = 100/(P_(1)) = 100/(-5) = -20 cm`.
Lens must be be concave.