Optical prisms are optical instruments comprising a block of glass, or other transparent material, in the shape of triangular prism, used to observe the phenomenon of light refraction.
The two most common prism models have, respectively, an angle of 60° or 45°.
When a beam of white light passes through a prism with 60° opening, its constituent colours are refracted in different directions, so you look out a succession of coloured bands, known as a spectrum.
This phenomenon is known as dispersion, and is due to the property of the various components of light (the colours), which are electromagnetic waves of different wavelengths, which behave differently when passing through glass: red light undergoes less slowdown compared to violet light.
Experiments conducted with prisms by the English scientist Isaac Newton led him to conclude that light coming from the Sun is comprised of all the colours in the spectrum, as seen in a rainbow.
The angle at which a ray of light leaves the prism after undergoing double refraction depends on the angle of incidence; by choosing the latter value you can ensure that the light beam, passing from glass to air, forms an angle with the second face of the prism below a certain value, called the critical angle.
In this case we can observe the phenomenon of total reflection: the prism works like an extremely efficient mirror.