Introduction

Procedure: Bubbles

• Gather these items from your home or workplace:
  • liquid detergent or glycerine
  • a metal ring through which to blow bubbles
  • two small glass slides

• Use the soap and ring to blow bubbles (preferably outside -- sunlight works best). What do you notice about the colors and patterns on the surface of the bubbles?

• Place one glass slide on top of the other and hold the two in bright indoor light or sunlight. Do you see any colors or patterns coming from the two slides? If not, try separating the slides by a tiny amount.

• Are the colors and patterns from the slides similar to those on the bubbles? Think of how these two seemingly very different experiments can yield very similar phenomena.

Procedure: Young's Fringes

• Gather these items from your home or workplace:
  • aluminum foil or a painted glass plate
  • a sharp knife or razor blade
  • a light source
  • a long strand of hair (try both thin and thick)

• Bring your thumb and index (first) finger close to one eye and pretend like you're trying to hold a needle. Keeping your thumb and finger separated, look at a light source through the tiny space between them. You should see tiny fringes very near the skin. Why does this diffraction pattern form?

• Make two thin slits, less than a millimeter apart, on the aluminum foil or on the glass plate. Make parallel slits of various separations, as well as a single slit, well separated from the rest, for comparison. (If you cut your pair of slits in the shape of a narrow wedge, you will get the effect of different spacings by looking through different parts of the wedge.)

• Pick a small concentrated light source: a bare bulb with a long filament (or a slit mask in front of an ordinary bulb) in a dark room, or a distant streetlight. Orient the slits parallel to the longest dimension of the light source. Place your eye close to the slits and focus on the light source. You will see a combination of Young's fringes and the single-slit diffraction pattern. The larger pattern is the diffraction pattern. You can identify it by comparing the pattern with what you see by looking through the single slit. Young's double-slit fringes are the fine lines in the central part of the diffraction pattern. The narrower, more uniform, and closely spaced your slits are, and the smaller and more distant your light source is, the better will be the fringes you observe. Experiment until you see good fringes.

• Using your narrowest slits, note the sequence of colors in the fringes. Estimate how many fringes you can see with white light, then try a color filter anywhere in the light path and see whether more fringes are visible. Verify that the fringe spacing changes with slit separation as advertised. If you have several color filters, switch them back and forth to see how the fringes move as the dominant wavelength changes.

• Take a strand of hair and hold it tightly (but not enough to break it) and vertically using both hands. Locate a distant bright light source (lamp bulb, street light) and look at the light with the hair superimposed on the source. Hold the hair close to your eye for best effect. The strand of hair acts as an "anti-slit"!