Question

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

• A.
• B.
• C.
• D. None of these

Answer 1

Answer: (A)

$m>\sqrt{2}$
$\Rightarrow \frac{1}{m}<\frac{1}{\sqrt{2}}$
$\Rightarrow \sin C<\sin 45^{\circ }$
$\Rightarrow C<45^{\circ }$
Hence a ray striking a glass-air surface from inside the prism will be reflected back if angle of incidence is not less than $45^{\circ }$
$90^{\circ }$ deviation is produced if the ray strikes on one of the sides normally as shown.
Here, $I=45^{\circ }$ and hence reflection takes place.