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Speed of light

Equipment set-up

The LED used

Control box with built-in photodetector

Light passing through a water filled tube

The speed of light is one of the most important and fundamental constants in physics and has been exactly defined since 1983. Throughout history, many attempts have been made to measure it; an early attempt by Galileo proposed a method by shining flashes of lights between adjacent hills a few kilometres apart. However, as the speed of light is so large relative to the distances used, the time intervals were too short to be measured by the equipment available at the time. Since then, technology has improved, and it is now possible to make analogous measurements within the confines of a laboratory. 

In this experiment, the light from a laser will be modulated at a fixed frequency, which changes the laser light intensity regularly. By changing the optical path along which the laser light travels, the variation of the phase intensity can be measured using an oscilloscope. 

Current students can find more information on Learn Ultra. 

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