Question

A double convex thin lens made of glass (refractive index $1.5$) has both radii of curvature of magnitude $20cm$. Incident light rays parallel to the axis of the lens will converge at distance $L$ such that

• A. $L=20cm$
• B. $L=10cm$
• C. $L=30cm$
• D. $L=40cm$

Answer 1

The correct option is A $L=20cm$

The point where parallel beam converges is focus.
hence we need to find focal length

using lensmakers formula for equiconvex lens,
$\frac{1}{f}=(\mu -1).\frac{2}{R}$
Placing $R=20cm$ and refractive index as $1.5$,
$\frac{1}{f}=(1.5-1).\frac{2}{20}$
$f=L=20cm$.