Question

A beam of light consisting of red, green and blue colours is incident on a right angled prism shown in figure. The refractive indicies of the material of the prism for the above red, green and blue wavelengths are $1.39,1.44$ and $1.47$ respectively. The prism will

• A. separate all the three colours form one another
• B. separate part of the blue colour from the red and green colours
• C. separate part of the red colours form the green and blue colours
• D. All the three colours will not emerge out from the prism

Answer 1

The correct option is C separate part of the red colours form the green and blue colours

Here incident angle is ${45}^{\circ }$. For total internal reflection, incident angle should be grater than the critical angle.

Assume incident angle is critical angle.
$\mu =\frac{1}{\sin c}=\frac{1}{\sin {45}^{\circ }}=1.414$
The colour will suffer total internal reflection for which the refractive index is less than $1.414$.
Therefore, red light will be refrated at the interface $CD$ where as blue and green light will get internally reflected.