Why does a prism create a spectrum of colours?

A prism creates a spectrum of colours due to the refraction and dispersion of light.

When a beam of white light enters a prism, it is refracted, or bent, and then refracted again when it exits. This bending of light is due to the change in speed of light as it moves from one medium (air) to another (glass). However, not all colours of light are refracted by the same amount. This is because each colour, or wavelength, of light has a slightly different speed in the glass of the prism. This phenomenon is known as dispersion.

The dispersion of light into its constituent colours is what creates the spectrum. The shortest wavelengths (blue and violet light) are refracted the most, and the longest wavelengths (red light) are refracted the least. This is why the colours spread out into a spectrum, with violet on the bottom and red on the top.

The concept of dispersion can be further understood by considering the refractive index of the prism. The refractive index of a material is a measure of how much it can bend light. It is dependent on the wavelength of the light, with shorter wavelengths being slowed down more and hence having a higher refractive index. This difference in refractive index for different wavelengths is what leads to dispersion and the formation of a spectrum.

In summary, a prism creates a spectrum of colours because of the way it refracts and disperses light. The different colours of light, each with their own wavelength, are refracted by different amounts due to their different speeds in the glass. This spreading out of colours, from red to violet, is what we see as a spectrum.