A point object is placed at a distance of 25 cm from a convex lens. Its focal length is 22 cm. A glass slab to 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).

When object is placed at the focus of the lens, i.e., at 22 cm from the lens, image will be formed at infinity. 

Shift in the position of object  should be = 25 -22 = 3cm

$$\begin{gathered} Shift = \left(1-\frac{1}{\mu }\right)t \\ \Rightarrow 3= \left(1-\frac{{\displaystyle 2}}{{\displaystyle 3}}\right)t \\ \Rightarrow t=9cm \end{gathered}$$

Because of the glass slab virtually the object lies at principal  focus of the convex lens.