Show with the help of ray diagrams that a right angled isosceles prism can produce a deviation of 90° If the refractive index is greater than $\sqrt{2}$.

$μ>\sqrt{2}$

$\frac{1}{μ}<\frac{1}{\sqrt{2}}$

$⇒ \sin C < \sin 45°$

$⇒ C < 45°$

Hence a ray striking a glass-air surface from inside the prism will be reflected back if angle of incidence is not less than 45°.

90° Deviation is produced if the ray strikes on one of the sides normally as shown.

Here, 1 = 45° and hence reflection takes place.