Breathe on a glass plate, then sprinkle a fine layer of lycopodium powder onto the plate. Light from a bright point source is focused by a 50 cm focal length convex lens into a hole in the front of a box, and the aperture is adjusted so that the light (without the scattering powder plate) just passes through the hole. When the powder plate is positioned in the beam, the diffraction is sufficient to create a colored halo around the hole, as pictured above. Using the average size of the lycopodium powder spheres (about 25-40 microns) the diffraction angle can be calculated approximately.

Actually this effect is a GLORY rather than a HALO; a halo is a refraction/dispersion phenomenon while a glory is a diffraction phenomenon.

There is a light diffraction ring with a diameter larger than the central scattering halo that is hard to see in the photograph above but which is readily apparent in the real thing.

Each spool has opaque material across both ends with a pinhole in each center. Viewing a distant light through the pinholes with the spool very close to your eye produces a nearly parallel light beam through your eye onto your retina. Red blood cells located directly in front of your retina diffract this parallel light, causing dark circle diffraction patterns on the retina.

These red blood cells sometimes cause floaters to appear in your eye when you stare into space, for example at the pure blue sky.

Drag a comb through the laser beam. The sawing sound is obtained by letting the laser beam strike a radiometer, the output of which is fed into an audio amplifier and loudspeaker. Cut off small segments of laser beam, which can be obtained by palming small pieces of previously prepared wire with red plastic insulation.

Do this one for elementary school kids. They will come up after your program to get a piece of the laser beam! The photograph at the right shows the comb used to saw the laser beam along with several pieces of beam excised in this manner.