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On the Fringe (formerly Bridge Light)

Science Snack
On the Fringe (formerly Bridge Light)
Air trapped between two pieces of clear plastic produces rainbow-colored interference patterns.
On the Fringe (formerly Bridge Light)
Air trapped between two pieces of clear plastic produces rainbow-colored interference patterns.

When light hits two slightly separated transparent surfaces, part of the light will be reflected from each surface. If the distance between the surfaces is a multiple of half the wavelength of any one color of light, destructive and constructive interference will occur, producing an interference pattern.

Tools and Materials
  • Two sheets of 1/8-inch (3 mm) clear plastic with smooth flat surfaces (no bumps or ridges, no burrs around the edges, not a lot of scratches, etc.), approximately 8 inches (20 cm) square (exact size is not critical)
  • Dark construction paper about the same size as the plastic
  • A bright light source, such as a desk lamp, bright overhead light or sunny window
  • A piece of transparent red plastic (not pictured) of any size (anything between 3 x 5 inches (8 x 13 cm) to 6 x 6 inches (15 x 15 cm) will work)
  • Optional: tape
  1. Clean the top and bottom surfaces of the plastic with window cleaner, alcohol, or soapy water and dry thoroughly. Avoid scratching or smudging the surfaces.
  2. Press the plastic pieces tightly together while holding the piece of dark construction paper under the bottom piece. NOTE: for convenience, you may want to tape the plastic pieces together and tape the construction paper in place. But do not put tape or anything else between the plastic pieces—the inside surfaces should press together as flatly as possible.
To Do and Notice

Hold the sandwiched plastic, with the dark piece of paper away from you, up to any strong source of white light. Observe the rainbow-colored interference patterns. The patterns will change as you bend, twist, or press on the plastic pieces. Notice that the patterns strongly resemble the contour lines on a topographic map.

Place the red plastic between the light source and the pieces of plastic. Notice that the patterns are now just red and black.

What’s Going On?

Light waves reflect from the surfaces of two plastic sheets separated by a thin air gap. These light waves meet after reflecting from the two surfaces. When two waves meet, they can add together, cancel each other, or partially cancel each other. This adding and canceling of light waves—called constructive interference and destructive interference—creates the rainbow-colored patterns you see.

White light is made up of all different colors mixed together. When light waves of a particular color meet and cancel each other, that color is subtracted from white light. For example, if the blue light waves cancel, you see what is left of white light after the blue has been removed, which is yellow, the complementary color of blue.

When you place a red filter in front of the light source, only red and black fringes will appear. Where destructive interference takes place, there is no red light left to reach your eyes, so you see black. Where the red light waves constructively interfere, you see red.

The thickness of the gap between the plates determines which colors of light cancel out at any one point. For example, if the separation of the plates is roughly equal to one-half the wavelength of blue light (or some multiple of it), the crests of waves of blue light reflected from the top surface of the air gap will match up with the troughs of waves reflected from the bottom surface, causing the blue light to cancel out.

This is what happens: Imagine that the distance between the two plates is one-half the wavelength of blue light. When a wave hits the top of the air layer, part reflects and part continues on. Compared to the part that reflects from the top of the air layer, the part that continues on and reflects from the bottom travels an extra wavelength through the air layer (half a wavelength down and half a wavelength back). In addition, the wave that reflects from the bottom is inverted. The net effect is that the blue light waves reflected from the two surfaces recombine trough-to-peak and cancel each other out. (See diagram; click to enlarge.)

Because the interference pattern depends on the amount of separation between the plates, what you’re actually seeing is a topographical map of the distance between plates.

Going Further

When you open a package of new, clean microscope slides, you can often see colored interference patterns created by the thin air space between the glass slides.

The beautiful rainbow colors you see in soap bubbles and on pieces of metal heated to high temperatures are produced in the same way: by light reflecting from the top and bottom of a thin transparent layer.