A small object is placed 10 cm in front of a block of glass 5 cm thick and its farther side silvered. The image is formed 13 cm behind the silvered face. The refractive index of glass is 

Due to refraction phenomenon the silvered surface CD is appears to be shifted i.e., ${\mathrm{C}}^1{\mathrm{D}}^1$as shown. According to properties of image formation with plane mirror OM=MI.
$\begin{array}{l}\Rightarrow 10+\mathrm{x}=(5-\mathrm{x})+13\\ \Rightarrow 10+\mathrm{x}=18-\mathrm{x}\\ \Rightarrow 2\mathrm{x}=18-10\\ \Rightarrow \mathrm{x}=\frac{8}{2}=4\mathrm{cm}\end{array}$
Refractive index $\mathrm{\mu }=\frac{\text{ real depth }}{\text{ apparent depth }}=\frac{5}{4}=1.25$