Measuring Speed of Light - Fizeau's Method

This section describes the method used by Armand Fizeau to measure the speed of light using using a rotating toothed wheel and a mirror.

In 1849, the French physicist Armand Fizeau created a new method to measure the speed of light more accurately using a rotating toothed wheel and a mirror, as illustrated in the picture below.

Fizeau's measurement is based on the following idea:

• Light coming from the source gets reflected through a rotating toothed wheel.
• The continuous light will be cut into chunks by teeth on the wheel.
• A chunk of light will pass through the gap between two teeth.
• That chunk of light will reach the mirror at a far distance (like 8,000 m) and reflected back.
• The reflected back chunk of light will be block by next tooth, the the wheel rotates at a slow speed,
• But if the wheel rotates at a certain fast speed, the reflected back chunk of light will pass through the gap after the next tooth. This rotation speed can be used calculate the speed of light.

Using this idea, Fizeau was able to measure the speed of light as 313,300,000 m/s. This is not very bad comparing today's definition of the speed of light: 299,792,458 m/s.

Last update: 2014.