Question

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

• A. $15cm$
• B. $-15cm$
• C. $5cm$
• D. $-5cm$

Answer 1

Answer: (A)

For final image at infinity, image shifted by the slab must be at the focus of convex lens.
Therefore, shift $=t\left(1-\frac{1}{\mu }\right)$
$\Rightarrow 5=t\left(1-\frac{2}{3}\right)$
$\therefore t=15cm$