rainbow

 
A wedge shaped volume of water can be used to separate sunlight into colored light. If you do this on a bright sunny day the colors you see will be quite vivid. Place a small mirror perhaps 2 by 3 inches in a shallow pan of water. Position so that sunlight comes in through a window, hits the part of the mirror that is underwater and is reflected onto the ceiling. This arrangement works well in the winter at 40 degrees latitude. For best results the light should be shown on a surface at least 4 feet away. Otherwise what you see is fringes of color at the edges of a spot of white light.
 
You can eliminate the smear at one edge of the patch of color. The smear results from the water's surface bending upward where it touches the glass of the mirror. Lay a piece of cardboard across the top of the pan so it blocks the appropriate light. If the spot of light is not separated into colors you may be looking at the wrong spot. Light will also be reflected off the water's surface and off the portion of the mirror that is out of the water.