Maxwell was able to deduce that light is electromagnetic wave as the speed of electromagnetic wave as predicted by his equations was very close to the experimentally calculated speed of light. He realized that this was unlikely a coincidence.

Experimental methods to calculate speed of light :

1. 1. Galileo
      • Galileo was one of the first to be recorded to conduct an experiment to determine speed of light.
      • He used to lamp on two hilltops few kilometres away.
      • The idea was he would open the shutter of the lamp and start measuring time.
      • His assistant would open the shutter of his lamp as soon as he saw Galileo’s lamp.
      • Galileo would stop timer as soon as he saw the assistant’s lamp.
      • He could only figure out that the speed of light is very high but not give any numerical value.
   2. Ole Romer
      • This Dutch astronomer was one of the first to give finite speed of light.
      • He measured the time for which the Io disappeared behind the Jupiter and noticed that these eclipses occurred earlier when the Earth was closer to Jupiter and later when the Earth was farthest away.
      • He explained his observations was due to the time taken by light to travel the extra distance when earth was closest to Jupiter and when it is farthest.
      • His calculations gave speed of time to be approximately 2.1 x 10⁸ ms^-1
   3. Fizeau

1. • Fizeau was one of the first to conduct a laboratory experiment to determine speed of light.
   • In his experiment, light passes through one notch of the rotating wheel and travels to a mirror about 8km away and is reflected back to an observer.
   • If the speed rotation wheel is adjusted, light can pass through without obstruction.
   • Considering speed of wheel and distance of mirror, he determined the speed of light to be 3.15 x 10⁸ ms^–
2. Leon Foucault
   • Foucault used a similar setup as Fizeau , but instead of using a wheel , used a rotating mirror.

• He determined the speed of light to be 2.98 x 10⁸ms^-1

5. Michelson

• Michelson used an eight sided rotating mirror to setup his experiment to measure speed of light.
• He used the speed of rotation and determined the speed of light to be 2.99796 × 10⁸ ms^-1

Currently the speed of light is defined as 2.99792458 x ms^-1

Currently the measurement of 1 metres is defined on the basis of the speed of light.

• 1 metre is defined as the distance light travels in
• where 1 second is defined as time taken for 9,192,631,770 transitions of Caesium-133 atom.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)