The oscilloscope shows the signal from a photo-detector when the laser (white, lower right) is modulated by a 1 MHz signal from the signal generator.

Pathlengths of 11.5 and 18.1 meters were used. Both gave velocities of about 2.3 x 10⁸ m/s for c. The measured times were 0.05 and 0.08 microseconds for the two cases.