The Armchair Experimentalist

The authors of texts often include little experiments to illustrate the subject. I consider these quite valuable, and not only perform those mentioned, but try to add more. In doing this, I often find that different things occur than envisioned by the authors. Physics is so clear and beautiful that it seems easy to predict what will happen without actually doing it. However, nature often injects some unexpected reality. This also happens in Astronomy, where I have seen ο Ceti, Mira, touted to the beginner as the brightest star in Cetus, apparently because it appears as a large circle on star charts and is sometimes listed as 2nd magnitude. However, the circle is an open one, and it also says that the star is variable. People who claim it as a celestial beacon have obviously never tried to see it, as I have (with success). Most of the time it lurks far beneath the limit of naked-eye visibility, only occasionally (about every two years for a few weeks) deigning to rise to 5th magnitude, and only very rarely any brighter. Many astronomical sights are described by those who have never actually seen them in the sky.

However, the stimulus for this article is a little experiment on the single-slit diffraction pattern from the renowned authorities Hecht and Zajac (See Reference). They recommend the line-filament incandescent bulbs, which will give bright spatially coherent light, as the source, and vernier calipers for a nice adjustable slit. One is confident from their learned description that they had actually done this, but on performing the experiment it is quite obvious that they had not. In the armchair, it seemed so straightforward as scarcely to require any verification.

It does, indeed, actually give what seems to be a single-slit diffraction pattern, a wide, bright peak at the middle. However, this is not all. There is a lovely, wide, double-slit diffraction pattern dominating the scene. The fine, equally-spaced fringes extend brightly and widely to both sides. This bonus is not mentioned by the authors, and perhaps has misled some less observant scholars. What happens is that the light encounters the flat, parallel surfaces of the caliper jaws at an angle of incidence near 90°, where they are perfectly reflective, and each projects a diffracted fan of light, because they appear very narrow to the incident light. There may even be some multiple reflection here that would narrow the observed fringes. These two bright lines are the source of the double-slit pattern. The fringe spacing should be measurable with a prominent visible scale at a known distance in the field of view.

For double-slit diffraction, the authors also recommend the messy process of coating a slide with graphite in alcohol, and scribing parallel lines. I doubt that they ever performed this very difficult process as well. Double slits can be made photographically with some ease, and many of us have done this. However, the best double-slit source is simply a piece of wire, which is absolutely parallel and of known size, and gives increased intensity. Thomas Young used a wire for his experiments. I can confirm that getting a double-slit pattern with a candle is not easy. Using narrow slits for diffraction is a bad idea, anyway, except for armchair diffraction, where they work perfectly.


E. Hecht and A. Zajac, Optics (Reading, MA: Addison-Wesley, 1979). pp 339-340.

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Composed by J. B. Calvert
Created 18 May 2003
Last revised