Question

A point object is placed at a distance of $25cm$ from a convex lens. Its focal length is $22cm$. A glass slab of refractive index $1.5$ is inserted between the lens and the object, then the image is formed at infinity. Find the thickness of the glass slab (in cm).

Answer 1

The optical length between the object and the lens changes .

The new optical length is : (real length between object and lens) - (real length of slab) + (apparent length of slab )
Given that the image is formed at infinity , so the object must be at the focal length .
Let the thickness be $t$ .
$\therefore 25-t+$ $\frac{t}{1.5}=f=22$

$25-\frac{t}{3}=22$

$t=9$