Question

A beam of light consisting of red, green and blue colours is incident on a right angled prism. The refractive indices of the material of the prism for the above red, green and blue wavelength are 1.39, 1.44 and 1.47
respectively. The prism will

• A. Separate part of red colour from the green and the blue colours
• B. Separate part of the blue colour from the red and green colours
• C. Separate all the colours from one another
• D. Not separate even partially any colour from the other two colours

Answer 1

The correct option is A Separate part of red colour from the green and the blue colours

At face AB, $i=0$ so $r=0$, i.e., no refraction will take place.
So light will be incident on face AC at an angle of incidence of ${45}^{\circ }$
The face AC will not transmit the light for which $i>{\theta }_C,$, i.e., $sin\left(i\right)>sin{\theta }_C$
or $sin\left({45}^{\circ }\right)>\left(\frac{1}{\mu }\right)$ i.e., $\mu >\sqrt{2}(=1.41)$
Now as ${\mu }_R<\mu$ while ${\mu }_G$ and ${\mu }_B>\mu$,so red will be transmitted through the face AC
while green and blue will be reflected. So the prism will separate red colour from green and blue.