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First it is important to understand what a prism is in this context. A prism is a block of material that conforms to these requirements:

* 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. This is because the prism is always denser (weighs more than the equivalent volume of) than the vacuum..

If the incident ray travels through a vacuum, then the angle of refraction is always greater than the angle of incidence. In this case, the ratio of the sine of the angle of refraction to the sine of the angle of incidence is called the refractive index of the prism material with respect to the wavelength of the incident ray. The ray of light in a vacuum travels at the "speed of light", c, as defined by Einstein's Theories of Relativity. In the medium of the prism, it travels somewhat slower. This is what causes the change of direction. The material of the prism, being denser than the vacuum, is more difficult for the ray of light to plow through. Think of a car with a blown front tire. It will pull to the side of the blown tire, changing the direction of the car. This is not a perfect analogy, but it might give you the idea. There are pretty diagrams available elsewhere (in some textbooks) showing how a wavefront changes direction when it slows down on an angle, but you will have to imagine that, or find one of those diagrams. See Snell's Law for the general case of the relationships of the angles of incidence and refraction, light velocities in each medium, and refractive indices.

Now we come to the nub of the matter. We have a ray of light traveling through the prism that must soon strike another of its surfaces. The ray emerges into the external medium (perhaps a vacuum) and the processes are reversed. The wavefront speeds up again to its former velocity (c in a vacuum) and keeps on going. If we assume that the surface it strikes is exactly parallel to the one whereby it entered, in this case the reversal will be exact and will produce the same angles as in entry, so that the ray will continue parallel to but offset from the course as followed before it entered the prism. This is not very exciting, though it is a way to offset the path of a ray of light. But if the exit surface is not parallel to the entry surface, the ray will shoot off in a quite different direction, having been bent twice (once at each surface), but not by the same amount each time.

The amount of bending depends on the refractive index. If we now think about what would happen to a ray of white (that is, not pure as regards wavelength) then its pure components will all shoot out in different directions. This is how a prism can resolve white light into a rainbow of colors.

See the related link for an interactive website dealing with rainbows, perhaps the best-known example of refraction. In this case the prism is a droplet of water embedded in air, and the ray is a ray of sunshine. The surface of the droplet is assumed to be spherical.

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βˆ™ 2013-05-20 12:28:49
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