Light changes speed as it moves from one medium to another (for example, from air into the glass of the prism). This speed change causes the light to be refracted and to enter the new medium at a different angle (Huygens Principle). The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media (Snell's law). The refractive index of many materials (such as glass) varies with the wavelength or color of the light used, a phenomenon known as dispersion. This causes light of different colors to be refracted differently and to leave the prism at different angles, creating an effect similar to a rainbow. This effect can be used to separate a beam of white light into its constituent spectrum of colors. Prisms will generally disperse light over a much larger frequency bandwidth than diffraction gratings, making them useful for broad-spectrum spectroscopy. Furthermore, prisms do not suffer from complications arising from overlapping spectral orders, which all gratings have.
But there is another way to look at the prism's light...
We are in a sense the prism. We started out as a hunk of ugly and distorted looking rock. Once God chose us to be one of His children he began chipping away at us. (As one of my College Professors stated from a book, "To be healed one must first be shattered."). He smoothed and polished us until we looked clear and beautiful, we then became the glassy prism. We went for awhile looking clean and sweet, but nothing ever came from us, nothing grand, nothing glorious. Christ then washed us with his blood and we became His children; it was now time for His glory to shine through us. Christ's light burst forth and shone brightly through our glassy figure. We were now hid from the world, and all they could see was the shimmering light from Christ hiding our form. Christ was ready to use us, and to show His power. Once God is using us others will soon see the vivid rainbow pouring through our lives, glimpsing the promise God has given. Much like the promise He set in the rainbow after Noah's flood, the prism's rainbow shining through us is also a promise to the lost world showing them that he is a loving God, yet He is also a righteous Judge. The lost world will find that God's promise for His children will never fail, and that at the end of our life we will resurrect and live eternity within His Kingdom. And at the same time they will find that there is a promise for those who do not follow after Christ, and this promise is spending eternity in hell-fire. But we have hope as Believers, God's promise will never fade, it as if the sun should never set and continued to shine through the prism forever, plastering the glorious rainbow throughout the world for eternity. God and His promise will never change, once we are His children He will continue to shine His light on us, and we must use this light to direct our prism, or body, towards pleasing Him. We must ask Him to show us where His light should be shown through us.