What it shows:

A point light source will produce seemingly sharp shadows which turn out to be not at all sharp when viewed under magnification. Narrow interference bands are seen within the shadow of a straight edge while more complicated shapes yield more complicated interference bands and striations.

How it works:

A point light source (spatially coherent light) is a necessity to see the effect. Although monochromatic light (temporally coherent ) produces sharper detail, white light works very well too. The most readily available light source meeting these prerequisites is, of course, the laser and the demonstration using a laser will be described first (it also works the best). However, dramatic effects can be produced with white light and these too will be discussed (2 & 3 below).

(1) It is necessary to diverge and expand the laser beam so that it illuminates an appreciable area of the object of choice - traditionally this has become a razor blade. Although a microscope objective can be used as a beam expander, by far the most satisfactory results are obtained using a spatial filter. ¹ The razor blade is positioned so that at least an entire corner and edge are illuminated; the closer to the laser, the larger the magnification on the screen, but less of the blade will be imaged. It's a compromise between blowing up the detailed interference pattern and still recognizing the object producing the effect. Ideally one would start with the object (razor blade) far from the laser (and thus totally illuminated) and then slide it close to the laser (on the optics rail) to blow up the pattern -- "animated" in this manner, the audience will have a better sense of what is being shown.

(2) Interference fringes can also be demonstrated with a point white light source. Our source is a small mercury arc lamp. It is, of course, quasi white light in the sense that it's a discreet multi-line source rather than a continuous spectrum and it's color balance is heavy toward the blue end. Nevertheless, it is not temporally coherent light. The arc lamp is placed far from the object (and screen) to more closely approximate a point source -- the arc is a couple of millimeters long. The object is positioned a couple of meters from the screen. The details in the shadow are quite sharp albeit dim and close-up video camera/projection is necessary for audience visibility. However, the real thing definitely looks better than the video image so, if it's a relatively small class, invite the students to come up and see for themselves after lecture.

(3) An alternative experiment dramatically demonstrates the diffraction of light around edges but does not show the structure of the interference patterns. Using the Schlieren Optics setup (refer to the description given) with most of the room lights turned off, place an object in front of the mirror. The audience will see (on the TV monitor or projector) a dark object brightly outlined by a thin boundary of light. This light, whose origin is the point light source, is being diffracted by the edges of the object into the eye of the observer -- in this case a video camera. The effect is quite striking indeed. The downside is that its not as pedagogically simple as the laser experiment ... there's a lot of explaining to do if the audience doesn't already understand Schlieren optics techniques.

Setting it up:

(1) Laser, beam expander, and object are mounted on an optics rail off to one side of the lecture hall while the projection screen is set up on the opposite side. This will give a large enough image for medium size classes. For larger classes, or if the lecturer desires to show greater detail within the shadow, use a video camera/projector to zoom in on the fine structure. Alternatively, the optics rail can be positioned at the back of the hall and the image projected on the large front screen. The greater projection distance will give an image big enough to be easily seen by even the largest audience. However, unless the lecturer is willing to run back and forth (in the dark lecture hall), an assistant will be necessary. The 35 mW HeNe laser gives a very bright image.

(2) Place the mercury arc source and power supply on a small cart off to one side of the lecture hall. Shield the lamp from direct view of the audience as it is not only annoying but, more importantly, dangerous (UV radiation). The lecturer should avoid looking at it too. The object whose shadow is to be cast on the screen sits on the lecture bench and is held with a lab clamp. The screen (a sheet of white paper will do) is placed one or two meters beyond the object. A video camera equipped with a 50 mm lens and 5 mm extension ring is positioned as close to the screen as practical.

(3) The Schlieren Optics setup is fully explained in its write-up. The optical alignment is quite involved and requires considerably more setup time than the 10 minutes or so between classes.

Comments:

Although not understood as a wave interference phenomenon, Francesco Grimaldi was the first to study and record these patterns of light within the shadows of objects in the 17 century.

1 A spatial filter consists of a very short focal length lens that focuses the laser beam through a pinhole. Being literally a point source of light, the beam strongly diverges from the pinhole. One usually follows this with additional optics to produce a parallel beam of light, or to focus the laser beam at some distance. Here we purposely dispense with that optics.