Question

A double convex thin lens made of glass has both radii of curvature of magnitude $20cm$. Incident light rays parallel to the axis of the lens will converge at a distance $L$ such that ______

Answer 1

Parallel rays converge at its focus.

From lens maker's formula,

$\frac{1}{f}=(n-1)(\frac{1}{R_1}+\frac{1}{R_2})$

Since $R_1=R_2=20$ cm

& material is glass, $n=1.5$

$\therefore$ $\frac{1}{f}=(1.5-1)\times \frac{2}{20}$

$\therefore$ $f=20$ cm.