A laser light pulse a few nanoseconds long is emitted by a light-emitting diode and immediately strikes a partially silvered mirror. The reflected light returns to a phototransistor to give the first pulse of the oscilloscope trace shown in the picture. The light transmitted through the partially silvered mirror reflects off a distant front-surface mirror, in the foreground of the picture, returning to the phototransistor to create the second pulse. The distance the laser beam travels, measured using a metric tape, and the elapsed time, determined from the oscilloscope trace, are used to calculate the speed of light.
Please Note: Care must be used to position the light reflected from the distant mirror onto the phototransistor. This needs to be carefully monitored while in use, as environmental changes can affect it. Also, if there are issues of signal stability, check the trigger level on the oscilloscope to confirm that it is compatible with the trigger signal.