While cleaning up some lab equipment, I accidentally came across a novel demonstration of light interference using a simple laser pointer and a glass beaker. As I passed a beam of laser light through the lip of a 500-ml glass beaker, I noticed the beam spread out into a “line” of light as it hit a nearby wall. The cross section of the lip is circular, so it acts like a cylindrical lens to the incident laser beam. As the beam passes through the lip, it refracts, converging and then passing through a focal point. It is in this manner that the beam fans out into a “line” rather than a dot when shining on the wall, as seen in the diagram in Fig. 1.
Surprisingly, when the line of light is cast onto a wall (30 cm from the beaker lip) a series of interference fringes can be...