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 $R.I.1.5$ is inserted between the lens and the object, the image is formed at infinity. The thickness $t$ of the glass slab is

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

Answer 1

The correct option is A $15cm$

When the object is placed at the focus of the lens, the image will form at infinity i.e. 20cm away from the lens. We will find the shift which is equivalent to the following relation:

$Shift=25-20=(1-\frac{1}{\mu })t$

$5=(1-\frac{1}{1.5})t$

$t=15cm$