Question

A thin convex lens $\left(L_1\right)$ of focal length $80$cm and a thin concave lens$\left(L_2\right)$ of focal length f are kept co-axially, $20$cm apart as shown in figure. When a narrow and parallel beam of light is incident on the convex lens, beam emerging from the concave lens is also a parallel beam. Find f.

Answer 1

Parallel rays after refraction through convex lens will converge at focus. So, convex lens will focus at $80$cm.

So, object for concave lens will be at $60$cm from it on the right hand side.

We know,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{\infty }-\frac{1}{60}$
$\Rightarrow f=-60$cm.
So, focal length of concave lens is $60$ cm.