Introduction: Planets and fringes Seeing interference fringes at home Seeing fringes with your telescope

For this experiment, you will need:

1. A laser pointer (available from some discount, stationery, and camera stores or by mail order for $7.00 - $70.00. The commonly available 5 mW, Class III red laser is preferred for safety reasons but green works better if the room cannot be darkened well).
2. A small piece or scrap of second surface mirror (the kind you find in a bathroom mirror, where the mirror's glass is between your eyes and the reflecting surface; flat first surface mirrors work as well). Alternatively, heavy aluminum foil and a clear piece of glass can be used.
3. Binder clips from a stationery store.
4. A single edge razor blade from a hardware store.
5. A ruler or straight edge.
6. A projection screen, or white wall or sheet of paper for use as a projection screen (a glossy surface should be avoided).
7. A flat work surface.

Use a razor blade and straight edge to make a straight, clean, narrow scratch (scrape, really) that removes the paint from the back of the second surface mirror but doesn't scratch the glass. Then make a second one parallel and a millimeter or less from it. If you are not successful, just try again: start in a corner of your mirror so you have practice room until you make a good single slit and a good pair of slits. Be careful of sharp edges when working with the blade and the glass.

Alternatively, heavy aluminum foil can be used. Cut slits through it with the blade, using cardboard to both protect the table top and permit penetration of the foil. Don't push down too hard with the blade: a narrow slit is desired. It may be helpful to stick cellophane tape, for added strength, on the foil before cutting two slits close together. You may wish to wrap, carefully, the aluminum foil around a clear piece of glass after the slits are made. (In use, send the laser beam through the foil first.)

Use a pair of binder clips to serve as legs supporting the laser and to hold its switch on. Use another pair of clips to hold up the piece of second surface mirror with the slits. Adjust heights so the laser beam passes through the slit(s). See Figure 1.

In a dark room, aim the laser beam at the painted side of the mirror so that the beam projects through the mirror onto a wall (or screen). Send the beam through a single slit to make a single slit diffraction pattern. This linear pattern is like a section through the bull's eye-like Airy pattern seen by astronomers looking at a star image through a telescope using high magnification. You can see examples in Figures 2, below. (You can use a needle to make a small hole in aluminum foil. Project the laser beam through the hole to create an Airy pattern on the screen. Beware of laser reflections from the aluminum foil going into eyes.)

Figure 1. The laser beam passes through the mirror from its painted side onto the screen. Binder clip "legs" support the laser and slits (mirror) on the table.

Now project the laser beam through the double slit to make an interference pattern on the wall. This fringe pattern is different, with more bright spots (maxima) and dark separators (minima) along the line of light. A careful examination may reveal that the spots are modulated by (follow the shape of) a single slit pattern: the brightest maxima are in the middle of the line. Looking outward from the bright middle in either direction, the maxima get fainter, to some minimum, and then get brighter again, and then fade away. (The room has to be dark enough and the laser bright enough or the screen close enough for this to be visible. Sometimes the beams from inexpensive laser pointers are not good enough to make a good double slit pattern.) Figure 3 shows an example of a double slit pattern.

Figure 2. Different shutter times, shorter (top) and longer (bottom) were used to photograph these single slit patterns. If a pinhole had been used instead of a slit, the patterns would look like a bulls-eye target - the Airy pattern seen by astronomers in their telescopes.

This is a double slit interference pattern made with an inexpensive laser. Note how there are additional maxima and minima of brightness, caused by the interference of light coming through the two slits. The single slit interference pattern modulates the brightness of the double slit pattern.

You can also try looking directly through single and double slits at a tiny, high intensity flashlight bulb (reflector removed from the flashlight) to see these patterns. Also, look at a light emitting diode (LED) at various distances from you to see at what distance the fringes disappear. Look at LEDs of different colors to see differences in the spacings of the fringes. DON'T EVER LOOK INTO A LASER BEAM - your eyesight can be damaged permanently.

An example is http://schools.matter.org.uk/Content/Interference/ laserinterference.html that permits you to change slit width, slit separation, and projection distance.

The website http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslidi.html graphs the brightness of the interference pattern (brightness is left-right, distance along a screen is up-down) for one slit or two slits (or more) and allows you to include diffraction (Airy pattern) or not in the interference pattern by clicking on a button. It shows, just barely, the secondary maxima and the striping you see with your set-up.