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 the object, the image is formed at infinity. The thickness $t$ is

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

Answer 1

The correct option is D $15cm$

For the image to form at infinity the object must be at focus of lens hence the shift is $25-20=5cm$

We know, $5=t(1-\frac{2}{3})$

$t=15cm$, Hence option $D$ is correct