Bridge Light lets visitors make interference patterns out of thin air. Two glass plates form a sandwich with a thin layer of air in between. By pressing on the top plate, bright and dark bands spread out in a swirling pattern. A bright band appears wherever light waves reflecting from the back and front surface of the air film between the plates coincide; a dark band appears where the light waves cancel each other out. The light source is a sodium vapor lamp and produces a pure yellow light of similar wavelengths, making visible the interference patterns that are not visible in white light, with its mixture of different wavelengths. The exhibit also illustrates absorption and reflection. Several brightly colored cards look dull under the sodium light, but appear in full color under white light.