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Equipment set-up

The LED used

Control box with built-in photodetector

Light passing through a water filled tube

Speed of Light

In this session you will have:

Learnt how to measure the speed of light


Practiced using an oscilloscope to measure phase shifts


Understood Lissajous figures


Demonstrated directly that light travels more slowly through water than air


Investigated the relationship between the refractive index and the speed of light in transparent media


This experiment measures one of the most important and fundamental constants in physics - the speed of light. You will measure it in air and in water and experimentally verify that the speed of light slows in transparent media. You will measure the ratio of the speed of light in a vacuum to that in a medium and thereby determine the index of refraction.

Fig. 1: Red light from an LED is modulated with a 60MHz signal and passed through a tube of water onto a photo-detector.


The script for this experiment can be found in the lab script book or on DUO.