PURPOSE: To measure the speed of light.

DESCRIPTION: 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 tra vels, measured using a metric tape, and the elapsed time, determined from the oscilloscope trace, are used to calculate the speed of light. The separation of the two light pulses in the picture is about 16 nanoseconds.

SUGGESTIONS: A few percent measurement can be readily attained with about 20 feet between the apparatus and the mirror. Care must be used to position the light reflected from the distant mirror onto the phototransistor.


EQUIPMENT: Speed of light module with oscilloscope, front surface mirror on moving stand, metric tape.

SETUP TIME: 5 min.

Go back to Lecture-Demonstration Home Page.