In the late 1960s, Arthur Schawlow, co-inventor of the laser, visited the author’s high school physics department to share some intriguing laser demonstrations. He popped colored balloons inside transparent ones, and showed interesting diffraction patterns with laser light. The most memorable demonstration began with the following question: “Can the wavelength of laser light be measured using only a common ruler?” Schawlow’s explanation can be simplified and used to easily measure the wavelength of laser light in the classroom (Schawlow 1965), which is described here.