Question

A CONCAVE LENS IS KEPT IN CONTACT WITH A CONVEX LENS OF FOCAL LENGTH 20 cm THE COMBINATION WORKS AS A CONVERGING LENS OF FOCAL LENGTH 100 cm CALCULATE POWER OF CONCAVE LENS.

Answer 1

Dear student,

Given:
Focal length of convex lens, f₁ = 20 cm = $\frac{1}{5}$m
Focal length of concave lens, f₂ = ?
Focal length of combination, f = 100 cm = 1 m

We know power of combination of lens is given as
$$\begin{gathered} P=P_1+P_2 \\ \Rightarrow \frac{1}{f}=\frac{1}{f_1}+P_2 \\ \Rightarrow P_2=\frac{1}{f}-\frac{1}{f_1}=1-\frac{1}{{\displaystyle \frac{1}{5}}}=1-5=-4\mathrm{D} \end{gathered}$$

Regards