A slide contains an array of 1, 2, 3, 4, and 5 slits. The single and double slit patterns are familiar. For N slits, where N is 3 or more, N-2 maxima of lesser intensity appear between the primary maxima. As N increases, the primary maxima increase in intensity as the 1st, 2nd, 3rd, 4th,etc. order spots, and these lesser maxima decrease in intensity until they cease to exist for a grating.

The picture below is for a three slits, so it shows one minor peak between each pair of major peaks.

Recommended primarily for small groups.

This is one of the keystone experiments in establishing light as a wave (rather than particles). The laser beam is expanded to around 4 cm diameter and passed around a 1 inch ball bearing which is suspended between two pointed rods. The diffraction pattern with its central bright spot is viewed on a distant screen. In the photograph at the left above the alignment of the ball in the expanded laser beam can be seen on the screen. In the photograph at the right the Poisson bright spot is seen on a ground glass screen about twenty feet from the laser, looking back toward the laser beam. This picture can be displayed on a video monitor or using a video projector.

There is a fascinating story about the origin of this experiment, referenced from Eugene Hecht, Optics (Second Edition) and a the web site Fresnel Diffraction, written by Dean Dauger:

In 1818, Augustin Fresnel submitted a paper on the theory of diffraction for a competition sponsored by the French Academy. His theory represented light as a wave, as opposed to a bombardment of hard little particles, which was the subject of a debate that lasted since Newton's day. Siméon Poisson, a member of the judging committee for the competition, was very critical of the wave theory of light. Using Fresnel's theory, Poisson deduced the seemingly absurd prediction that a bright spot should appear behind a circular obstruction, a prediction he felt was the last nail in the coffin for Fresnel's theory. However, Dominique Arago, another member of the judging committee, almost immediately verified the spot experimentally. Fresnel won the competition, and, although it may be more appropriate to call it "the Spot of Arago," the spot goes down in history with the name "Poisson's bright spot" like a curse.

Note that the alignment of this system can be delicate and time-consuming; it is not recommended to combine with with other demonstrations using the same laser in a single 50-minute lecture.

Breathe on a glass plate and sprinkle the plate lightly with lycopodium powder to create diffraction sources for the laser light. Laser light shining around the tiny spheres of lycopodium powder is diffracted, creating a series of rings around the main spot on a distant screen. The average diameter of lycopodium powder spheres is about 25-40 microns.

Actually, this is a GLORY, not a HALO; a halo is a refraction/dispersion phenomenon while a glory is a diffraction phenomenon.

Location of the screen determines the nature of the diffraction pattern: for nearby screen Fresnel diffraction is obtained, while for a distant screen Fraunhofer diffraction is obtained.

Lens L1 (very close to the diffraction obstacle) maps the entire region to the right of the aperture to within the focal length of the lens. Lens L2 projects one plane at the focal point of the lens onto a screen. Moving L2 from about f2 away from L1 to f1+f2 away from L1 projects successively every plane, from the aperture to infinity, onto the screen. Thus as lens L2 is moved the diffraction pattern changes, as if the screen starts close and moves to infinity, showing first Fresnel diffraction, the transition region, and ending with Fraunhofer diffraction.

The Poisson bright spot can be seen with the opaque disc if alignment along the optic axis is reasonably accurate.

The focal lengths fm of a Fresnel zone plate are given by the relation fm=R1^2/mL, where R1 is the radius of the first zone and L is the wavelength of the light used. A small magnet-mounted diverging lens is placed on the end of the laser, the Fresnel zone plate and a 20 cm focal length convex lens are mounted on a single holder, and a 2 cm focal length convex lens is mounted on a second holder such that it can be moved very close to the first lens. The screen is opposite the laser on a two-meter optical rail.

Start with the movable lens about 60-65 cm from the screen, and slide it back toward the laser. The first focus you come to (about 68 cm from the screen) corresponds to m=0, the zeroth order of the zone plate. Then, m=1 is at 83 cm, m=3 at 93 cm, m=5 at 97 cm. Between these foci, there is a small, bright "focus" which forms in a manner different from the others. These occur at 89 cm, 96 cm, etc., and correspond to the even orders m=2, 4, 6, etc. Ideally they should not be present because at these distances each transparent annulus contains an even number of Fresnel zones, which should exactly cancel out. That is, the "single slit" diffraction should cancel the interference, just as in the linear diffraction grating. If the rings aren't drawn perfectly, the cancellation isn't perfect, so the even orders are seen, as in the linear grating with slightly different spacing.

Using this technique we find that the measured R1^2/L is 64.1 cm, not 2.3 m as specified on the zone plate. This gives R1=0.64 mm, which is quite accurate.