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 or whole wavelengths of the light, constructive and destructive interference will occur, producing an interference pattern.

• Materials
  • 2 microscope slides or two acetate plastic sheets about 1 x 3 inches
  • 1 piece of dark construction paper
  • masking tape
  • 1 piece of transparent red acetate plastic, 1 x 3 inches
  • extended light source,
    
• Assembly
  Make sure the microscope slides are very clean. Use warm water and detergent to clean them, if necessary. When they are very dry, press them carefully together and tape the ends to hold them in this position. Tape a sheet of dark construction paper to one side to make the interference patterns more visible.
  
• Explore and Learn
  • Hold the slides, with the dark-paper side on the bottom, in any strong source of white light. Observe the rainbow-colored interference patterns. The patterns will change as you bend, twist, or press on the slides with your fingers. Notice that the patterns strongly resemble the contour lines on a topographic map.
  • Place the red plastic between the light source and the plates. Notice that the patterns are now just red and black.
    
• What's Going On
  Light waves reflect from the inside surfaces of the two glass slides 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 that 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--yellow (the complementary color of blue).
  
  The thickness of the gap between the slides determines which colors of light cancel out at any one point. For example, if the separation of the slides 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 slides 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.
  
  Because the interference pattern depends on the amount of separation between the slides, what you're actually seeing is a topographical map of the distance between slides.
  
  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 waves constructively interfere, you see red.
  
• Connections
  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.
  
  One of the more common places that you'll see this effect is in an oil slick. This is a layer of oil on the surface of water. It's common to see it in a parking lot (where cars contribute the oil) on a cloudy day (where the light is coming from all directions). You can't see the colors on a sunny day, since light reflection at a variety of angles is needed to see the effect.