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

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

PI₂ = µ(PI₁) = $\frac { 3 } { 2 }$ × 20 = 30 cm

So the rays will converge at a distance of 40cm from the lens