• The refractive index of the prism glass was assumed to be 1.51 for red light (≒700 nm) and 1.53 for blue light (≒400 nm).

The speed of light is about 300,000km/s. Light travels fastest in a vacuum. As light passes through the material, the speed decreases slightly depending on the material.
The speed of light reduction depends on the type or density of the material.

Suppose the speed of light is ‘1’ in a vacuum, then the speed is reduced to ‘\(\frac{1}{n}\)’ as the light passes through an object. The value of ‘\(n\)’ is called the refractive index of the object.

Even with the same material, the refractive index varies slightly depending on the wavelength.
For example, a red light (wavelength 700 nm) has a refractive index of 1.33 in water, while a purple light (wavelength 400 nm) has a refractive index of 1.34 in water.

Because each light has a different speed, it is differently refracted, so the distortion degree varies. It is called a spectrum that the color separates the light according to the wavelength.