Question

A point object is placed at a distance $25cm$ 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, image is formed at. Thickness $t$ is found to be $K$ times of $5cm$. Find $K$.

Answer 1

For the image of form at infinity by a convex lens, the object to be kept at first principal focus. Thus the insertion of the glass slab produces and image of the original object at the first principle focus which acts as object for the convex lens.

Thus the apparent change in position of object =$t(1-\frac{1}{\mu })$

$25-20=t(1-\frac{2}{3})$

$t=15cm=5K$

Thus $K=3$