CBSE Class 10 Science 2019 · Paper 31/3/2 · Question 5

White light is a mixture of colours, and glass bends (refracts) each colour through a slightly different angle: violet the most and red the least. This happens when light enters a prism and also when it enters a rectangular glass block. In a rectangular glass block the two refracting faces are parallel. The extent of bending at the second face is equal and opposite to that at the first face, so every colour emerges parallel to the incident ray (angle of emergence = angle of incidence) and hence parallel to the other colours. The colours, only shifted sideways, overlap and recombine into white light, so no spectrum is produced. In a prism the two refracting faces are inclined to each other, so the bending at the second face adds to the bending at the first. The different colours emerge in different directions, spread out and form a spectrum.