Deck prism

A deck prism is a special glass that some people place on boats to help light up the inside of their ship. It looks like a pretty 3-sided glass pyramid. You put it on the deck of the boat and it captures sunlight, much like a magnifying glass. When light enters into it, a beam of rainbow-colored light is cast across the room underneath the prism. This prism works so well that sailors can even use it to read in the dark! So it helps them to see when it is dark outside, and they do not need to use electricity to light up their ship. It's like having your own little rainbow on your boat!