Question

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

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

Answer 1

Answer: (D)

At a distance of $40cm$ from the lens

The lens will converge the rays at its focus, i.e., $30cm$ from the lens or $20cm$ from the refracting surface.
$P{I}_1=20cm$
$\therefore P{I}_2=\mu \left(P{I}_1\right)=\frac{3}{2}\times 20=30cm$
So, the rays will converge at a distance of $40cm$ from the lens.