A point luminous object (O) is at a distance h from front face of a glass slab of width h and of refractive  index  $\mu$. The back face of slab is a reflecting plane mirror. An observer sees the image of object in mirror

Distance of image from front face of the slab as seen by observer will be

As shown in figure glass slab with form the image of bottom i.e., mirror  $M{M}^1$ at depth $\left(\frac{d}{\mu }\right)$ from its front face. So the distance of object O from virtual mirror  $m{m}^1$ will be $h+\left(\frac{d}{\mu }\right)$

Now as a plane mirror forms image behind the mirror at the same distance as the object is in front of it, the distance of image I from $m{m}^1$  will be $h+\left(\frac{d}{u}\right)$  and as the distance of virtual mirror from the front as seen by observer = $[h+\frac{d}{\mu }]+\frac{d}{\mu }=h+\frac{2d}{\mu }$