Velocity of Light
11224.90 Velocity of Light Apparatus
The beam from a 50 MHz modulated LED is directed to a receiving diode through an optical path that is reversed by a pair of mirrors which can be moved to change the path length. The modulated received signal is phase shifted relative to the original signal as a function of the length of the light path. The phase relation is represented as a Lissajous figure on a two-channel oscilloscope. Both the emitted and the received signals are frequency shifted with a 1:1000 reduction, so that simple oscilloscopes and frequency counters having 1 MHz bandpass can be used. Receiving and emitting diodes are installed and aligned in the chassis. The apparatus has a measuring path of 1.5 m. With the assistance of a phase shifter, a defined phase relation (e.g. a Lissajous straight line) can be adjusted for a position of the mirror directly in front of the emitter receiver unit.
This apparatus was favorably reviewed in the "Physics Teacher" Phys. Teach. 35 295 (1997).