Question

ILION
length and nature of this combination.
20.. Two thin lenses of + 10 D and - 10 D powers are placed
in contact with each other. Find the power, focal​

Answer 1

Given,

Power of first lens is +10 D

Power of second lens is -10 D

To find,

Power and the focal length of the combination.

Solution,

The lens are placed in contact with each other. The power of the combination of lens is equal to the sum of individual lenses. So,

P = P₁ + P₂

We have,

P₁ = +10 D and P₂ = -10 D

So,

P = 10 + (-10) = 0

The reciprocal of power is equal to the power of a lens. So,

$f=\dfrac{1}{P}\\\\f=\dfrac{1}{0}\\\\f=\infty$

Therefore, the power is 0 and focal length is infinite.