Light passing through a prism can be split into exciting colours
In the Light and Color unit of The Physics Classroom Tutorial, the visible light spectrum was introduced and discussed. Visible light, also known as white light, consists of a collection of component colors. These colors are often observed as light passes through a triangular prism. Upon passage through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion. It was mentioned in the Light and Color unit that each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism. In this unit, we will investigate the dispersion of light in more detail, pondering the reasons why different frequencies of light bend or refract different amounts when passing through the prism.
Earlier in this unit, the concept of optical density was introduced. Different materials are distinguished from each other by their different optical densities. The optical density is simply a measure of the tendency of a material to slow down light as it travels through it. As mentioned earlier, a light wave traveling through a transparent material interacts with the atoms of that material. When a light wave impinges upon an atom of the material, it is absorbed by that atom. The absorbed energy causes the electrons in the atom to vibrate. If the frequency of the light wave does not match the resonance frequency of the vibrating electrons, then the light will be reemitted by the atom at the same frequency at which it impinged upon it. The light wave then travels through the interatomic vacuum towards the next atom of the material. Once it impinges upon the next atom, the process of absorption and re-emission is repeated.
The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. Thus, while a light wave travels through a vacuum at a speed of c (3.00 x 108 m/s), it travels through a transparent material at speeds less than c. The index of refraction value (n) provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void. The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction. A light wave would be slowed down to a greater extent when passing through such a material
What was not mentioned earlier in this unit is that the index of refraction values are dependent upon the frequency of light. For visible light, the n value does not show a large variation with frequency, but nonetheless it shows a variation. For instance for some types of glass, the n value for frequencies of violet light is 1.53; and the n value for frequencies of red light is 1.51. The absorption and re-emission process causes the higher frequency (lower wavelength) violet light to travel slower through crown glass than the lower frequency (higher wavelength) red light. It is this difference in n value for the varying frequencies (and wavelengths) that causes the dispersion of light by a triangular prism. Violet light, being slowed down to a greater extent by the absorption and re-emission process, refracts more than red light. Upon entry of white light at the first boundary of a triangular prism, there will be a slight separation of the white light into the component colors of the spectrum. Upon exiting the triangular prism at the second boundary, the separation becomes even greater and ROYGBIV is observed in its splendor.