A prism is a three-dimensional solid object. It is an optical medium that is bounded by three rectangular faces and two triangular faces parallel to each other. Two of the rectangular surfaces are polished, and it is called the refracting faces. The edge that is common to them is called the refracting edge of the prism. The angle between them is called the angle of the prism.
Dispersion through the Prism
Light of a single wavelength is called monochromatic light. For example, Laser light. The light consisting of a mixture of wavelengths is called composite light. For example, white light. Light of all wavelengths travels with the same speed in air or vacuum. The composite light of different wavelengths travels at different speeds in a material medium. As a result, the refractive index of a material medium is different for different wavelengths of light. They are said to exhibit dispersion. A medium in which the speed of light varies with wavelength is called a dispersive medium.
When a narrow beam of white light passes through the prism, the beam that emerges out of the prism is seen to have seven colours in the order violet, indigo, blue, green, yellow, orange and red. The band of colours that emerges from the prism is called the spectrum of white light. The phenomenon of splitting a ray of white light into its constituent colours is known as dispersion.
Refractive Index of the Material of the Prism
The refractive index is the measure of the bending of a light ray when passing from one medium to another. The refraction index of the material of the prism is given by
n = [sin (A + D)/2]/[sin A/2]
A is the angle of the prism
D is the minimum deviation
The refractive index is different for different colours. As a result, angles of refraction are different for different colours. Different colours deviate through different angles as light emerges out of the prism. Thus, different colours in the incident beam emerge from the prism in different directions. This accounts for the dispersion produced by a prism.
Angular Dispersion
The angular dispersion between two colours is the difference between their deviations.
Angular dispersion = (nv – nr) A
nv is the refractive index of violet light
nr is the refractive index of red light
A is the angle of the prism
The dispersive power of a medium for a pair of colours is the ratio of the angular dispersion between them to the mean deviation.
Do check the BYJU’S page for more Physics topics. Physics topics included are screw gauge, vernier caliper, refraction, reflection and much more. The vernier caliper is a measuring instrument that is used to measure linear dimensions. It is also used in the measurement of the diameter of circular objects. The least count of a vernier caliper is called the vernier constant. The zero error of the instrument is the condition when the vernier caliper registers a reading when there should not be any reading.
