: As beam oflight is incident normally on the face of the right angled prism so no refraction occurs at face and it passes straight and strikes the face at an angle of incidence,

For total reflection to take place at face

or

where ic is the critical angle.

But as here and

As

while and ,

so only red colour will be transmitted through face while green and blue colours will suffer total internal reflection.

So the prism will separate red colour from the green and blue colours as shown in the given figure.