In activity (3) and (4), we observe that white light is splitting into certain different colours.
In activity 3, we observe that the angle of deviation is minimum for red as compared to the angles of deviation of other colours and maximum for violet.
The splitting of white light into different colours (VIBGYOR) is called dispersion.
In our previous discussion, we learnt that for a particular refractive index of prism there must be only one angle of minimum deviation and according to Fermat’s principle, light ray always chooses the path of least time. But in activity-3,we noticed that light has choosen different paths.
The situation we witnessed in activities (3) and (4) rule out ray theory of light. We can consider that white light is a collection of waves with different wavelengths. Violet colour is known to have the shortest wavelength while red is of the longest wavelength.
According to wave theory, light can be thought of a wave propagating in all directions. Light is an electromagnetic wave. Here no particle physically oscillates back and forth. Instead, the magnitude of electric and magnetic fields, associated with the electromagnetic wave, vary periodically at every point. These oscillating electric and magnetic fields propagate in all directions with the speed of light.