The speed of light often cited in astronomy only holds true in a vacuum. When light enters water and other clear materials, it slows down a little which causes it to bend (refraction). The different colors that make up light slow down and bend differently, causing them to break into a spectrum (dispersion), as explained here.
It has been said that, "Life is stranger than fiction." That expression pops into my mind whenever I read about Sir Isaac Newton. Newton would still be a historic figure if he had only invented calculus at age 24, or if he had only articulated his three famous laws of motion. But Newton questioned everything! When he turned his attention to the nature of light and color, he single-handedly started yet another revolution when he asserted that white light was built of colors. Read this short BBC biography (a few pages) that includes a description of his experiments with prisms. Fascinating!
Note that Newton discovered the best spectrums are made with a slim beam of light hitting the prism and plenty of distance in a dark room for the colors to disperse (22 feet from the window to the wall opposite in Newton's experiments).
Here is an interactive tutorial that simulates what happens when you increase the beam width or change the angle it hits the prism. And here is another one that includes reflection within a prism (click on the light and move it).
Here is an interesting interactive site that combines optical physics and eye physiology. And here are some interactive pages about additive colors and subtractive colors. Here's the index of all the interactive java applets about optics on this Olympus site.
On either side of a the visible spectrum (red to violet) are invisible bands of the spectrum. Beyond the red is "infrared." Even though you can't see infrared, you feel it as heat. Check this out to see what photographing a star system with infrared film reveals. Here you can read how infrared was discovered and see the whole spectrum--visible and invisible.