Question

A point object is placed at a distance of 25 cm from a convex lens of focal length 20 cm. If a glass slab of thickness t and refractive index 1.5 is inserted between the lens and the object, the image is formed at infinity. The thickness t is

• A. 10 cm
• B. 5 cm
• C. 20 cm
• D. 15 cm

Answer 1

The correct option is D 15 cm

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
If the final image is at infinity, the image due to slab should be at the focus of the lens hence the apparent shift is 5 cm
Shift = 5 = $(1-\frac{1}{\mu })t$
t =15 cm