A prism is any object that separates white light into the colors of the rainbow -- red, orange, yellow, green, blue, indigo, and violet. It works by refracting light, and breaking it according to its wavelength. You can purchase a triangular prism of glass or plastic from hobby stores, and you can also make a prism from a simple glass of water.
Fill the glass with water so that it’s slightly more than half full. Place the glass on the edge of a coffee table or other flat surface, so that almost half of the glass’s bottom hangs over the edge. Be careful that the glass does not fall over the edge.
Place the two sheets of paper side by side on the floor next to the coffee table. Turn the flashlight on and point it toward the glass so that the light goes through the glass and onto the sheets of paper on the floor.
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Adjust the position of the flashlight and the paper until you can see the characteristic rainbow on the sheets of paper. This may require a trial and error to get the angles just right. You may also get more than one rainbow.
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Use the sun as an alternate source of white light. Set the glass on the edge of a window sill and adjust its position to get the sunlight to make the prism effect on the paper.
You can also make a prism using a CD. Poke a small hole in a piece of aluminum foil and fold the foil over the flashlight. Shine the flashlight onto the back of the CD. Or you can get a prism effect simply by holding the CD up to a light bulb so that the back of the CD faces the light bulb.