* Is transparent or at least translucent to light .
* Is not of the same composition as its surroundings (for example, there is little point in having a prism composed of water entirely immersed in water -- where would its boundaries be?)
* It may or may not have two (usually flat) faces not parallel to each other. The prism used by Isaac Newton when he first demonstrated the resolution of white light into its constituent colors more than 300 years ago is nearly always shown as being triangular in section.
It is also important to agree about what light is. As Einstein demonstrated in 1905, light may be considered either as a stream of particles (corpuscles), or as a bunch of waves. Here let us think of light as a bunch (pencil) of waves.
White light for instance contains light of many different wavelengths, just as by analogy white sound has sounds of many wavelengths. Thinking about the rainbow with all its colors, red light at one end of the visible spectrum has a longer wavelength than the violet light at the other end. All the colors in between have intermediate wavelengths.
Let us further consider a ray of light containing light of only one wavelength. This may be thought of as pure light. It can be of any color of the rainbow.
As the ray of pure light strikes a prism, two quite distinct things can happen. Either the ray is entirely reflected from the surface of the prism, or it is not. This depends on the nature of the surface of the prism.
The part of the ray that is reflected bounces off the prism as a ray in the plane defined by the striking ray (incident ray) and the perpendicular to the surface of the prism at the point of incidence. The angle between the incident ray and the perpendicular to the surface of the prism at the pint of incidence is called the angle of incidence. The angle at which the reflected ray leaves the surface, known as the angle of reflection, is equal to the angle of incidence, exactly the same as a ray might be reflected from a mirror. Indeed, for this reflected light, the surface of the prism is acting as a mirror.
If the ray is not reflected at all, or not completely reflected, then at least part of the incident ray of light actually enters the prism. This is said to be refracted. It enters the prism at an angle usually different from the angle of incidence. The angle of refraction (the angle between the ray and the perpendicular) depends on the angle of incidence, the wavelength of the incident light and on the material of the prism. It may be greater or less than the angle of incidence. If the incident ray travels through a vacuum, then the angle of refraction is always greater than the angle of incidence.
