The separation of colors by a medium is not a sufficient condition to observe dispersion of light. These colors must be widely separated and should not mix up again after emerging from the dispersing medium. A glass slab is not suitable for observing dispersion as the rays of the emergent beam are very close and parallel to the incident beam.
Newton used a prism to demonstrate dispersion of light. White light from a slit falls on the face AB of the prism and light emerging from face AC is seen to split into different colors. Colored patches can be seen on a screen. The face AC increases the separation between the rays refracted at the face AB.
The incident white light PQ splits up into its component seven colors: Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).
The wavelengths travelling with different speeds are refracted through different angles and are thus separated. This splitting of white light into component colors is known as dispersion. MR and MV correspond to the red and violet light respectively. These colours on the screen produce the spectrum.
The bending of the original beam PQN along MR and MV is known as deviation. The angle between the emergent ray and the incident ray is known as the angle of deviation. Thus, δv and δr represent the angles of deviation for violet light and red light, respectively.