Question

Focal length of a thin convex lens is $30 \,cm$. At a distance of $10 \,cm$ from the lens there is a plane refracting surface of refractive index $3/2$. Where will the parallel rays incident on lens converge?

• A. At a distance of 27.5 cm from the lens
• B. At a distance of 25 cm from the lens
• C. At a distance of 45 cm from the lens
• D. At a distance of 40 cm from the lens

Answer 1

Answer: (D)

The lens will converge the rays at its focus i.e., $30 \,cm$ from the lens or $20 \,cm$ from the refracting surface.

$P I_{1}=20 \,cm$
$\therefore P I_{2}=\mu\left(P I_{1}\right)$
$=\frac{3}{2} \times 20=30 \,cm$
So, the rays will converge at a distance of $40 \,cm$ from the lens.