An air bubble in glass slab  $(\mathrm{\mu }=1.5)$  when viewed from one side appears to be at 6 cm and from opposite side 4 cm. The thickness of glass slab is:

We know that $\mathrm{\mu }=\frac{\mathrm{real}\mathrm{depth}}{\mathrm{apparent}\mathrm{depth}}$

Let the thickness of the slab be t and real depth of the bubble from one side be x. Then   $\mathrm{\mu }=\frac{\mathrm{X}}{6}=\frac{(\mathrm{t}-\mathrm{X})}{4}\mathrm{or}1.5=\frac{\mathrm{X}}{6}=\frac{(\mathrm{t}-\mathrm{X})}{4}$$\mathrm{X}=9 \mathrm{and} 1.5=\frac{\mathrm{t}-9}{4} \mathrm{or} \mathrm{t}=15\left(\mathrm{m}\right)$