|Fill a glass of water (almost to the top)
and place it at the very edge of the counter in a dark kitchen.
Place a sheet of plain white paper on the floor a few inches away from the
counter. Put two pieces of masking tape over the front of a
flashlight so that the light comes out of a slit about 1/8 inch (0.3 cm)
wide. Shine this light across and down into the water as shown in
the figure. When a narrow beam of light is
passed through a glass of water, a spectrum can be seen on a white sheet
Can you see a small rainbow on the white paper? If not, move the flashlight around a little until you achieve the best results.
Now that you know what to look for, you will begin to notice small rainbows wherever sunlight falls on water, glass, or plastic that has a tapered shape. Look for rainbows produced by the beveled edge of a mirror or a chandelier.
The figure on the right and the explanation above were reproduced from Dr. Kurt Nassau's book, Experimenting with Color, 1997, with his kind permission.