My science fair project is on how prisms separate light. What is light made of?
At its most fundamental, light is made of photons, which are one of the basic particles which make up the universe. Our universe has two kinds of particles: "matter" particles, namely quarks and leptons, and "force carrier" particles (photons, gluons and vector bosons). For a friendly and thorough introduction to these particles, see The Particle Adventure.
Mostly - such as when we see something with our eyes - we're interacting with vast numbers of photons at one time. In those cases, we're dealing not with individual photons but with their behavior as a whole. Think of the beautiful, wave-like movements of a huge school of fish. Generally, you watch the overall pattern and don't pay attention to what any one fish is doing. It's the same with a "school" of photons. Mathematically, we treat it just like a wave. As a wave, light has a wavelength (the distance from one wave crest to the next). Light we can see (visible light) has a wavelength that ranges from 400 to 700 nanometers (nm for short). A nanometer is one billionth of one meter. (Here's an exercise - calculate your height in nanometers). The shortest visible wavelength is blue (400 nm) and the longest visible one is red (700 nm). The other colors fall in between just like in a rainbow.
When light shines through a prism, the glass changes the behavior of each light wave according to the wave's wavelength. This property of glass is known as its refractive index. The prism changes the direction the light is traveling. Shorter (blue) wavelengths bend more than longer (red) ones do. As a result, what goes in as ordinary white light comes out separated into a spectrum of different colors. That's because white light is really all those wavelengths mixed together. To shed more light on this subject (sorry, I couldn't resist), check out your local library. There are many interesting, not to mention colorful, books available for all ages.
Carl Zorn, Detector Scientist (Other answers by Carl Zorn)