Question

Why do lenses not disperse light and why do only prisms do the same? Both are able to refract light.
please please please please help me or I will be failed in exam :(​

Answer 1

Answer:

It the triangular shape of the prism which makes disperse possible.

When white light entering a prism with fixed angle of incidence at face-1, it is first refracted towards the normal (bending of ray) at the air-glass interface. Since white light consists of seven wavelengths or colors, each of these color light bend to different extent. When these seven color rays come out from prism at face-2, again there is a refraction, now the rays will bend away from the normal with large angle which makes us to see seven different colors.

In a piece of convex lens, due to the curvature, angle of incidence is different to diffident wavelength at the point of incidence hence after refraction all the seven color gets refracted to same angle merging all the colors. Hence there is no dispersion (spreading of light) at both the faces.

Answer 2

Hello

See in prism opposite faces are same whereas in lens opposite faces are not same. So the bending of light of different colours is different in both and hence only prism show dispersion.