Thursday, May 27, 2010

Using a ruler to measure the wavelength of light.

Place a ruler on your desk--the metal kind is best, but even a wooden ruler will do--and shine a laser pointer at its finest scale at glancing incidence. A diffraction pattern should appear on the far wall. A ruler is a reflection grating.

In the November 1965 issue of the American Journal of Physics, Arthur Schawlow published a brief article describing what a "lecture demonstration" of the possibility of measuring the wavelength of visible light with a ruler and presenting without derivation the formula relating the scale spacing on the ruler and the fringe spacing on the screen to the wavelength.

The widespread availability of cheap lasers has made this feasible as a student lab, or even as cocktail-party fare. I wouldn't recommend going through the geometry needed to compute spacing between fringes at a party, at least not until everyone else has had two more drinks than you, but it's something that can be done by students in the small-angle approximation (as by Schawlow) or possibly, using a computer, exactly.

I've had students measure their laser pointer's wavelength to within a nanometer of the specified value. Not bad for metrology done with standard office supplies!