The wheel that reveals the speed of light

Is there anything a wheel can't do? The Fizeau Wheel helped calculate the speed of light itself.

Just one year shy of 1850, French physicist Armand Fizeau gave the world The Fizeau Wheel, and with it a way of not only proving that light travels, but measuring the speed of its travel through the air. It's a relatively simple experiment, that requires only a candle, a mirror, some space, and a wheel shaped like a gear with widely-spaced teeth.


The Fizeau Wheel set-up is relatively simple. Take a candle, or other bright light source, and place it behind the wheel, with the wheel perpendicular to it. Well away from the wheel should be the mirror. The light from the candle should be able to travel through the slats of the wheel, hit the far mirror, and be reflected back to shine through the gaps in the wheel. The onlookers stand behind the wheel (near the candle) and look through the slats. (In the picture below, an angled half-silvered mirror is used to let the light through, and reflect light back through the tube to the onlookers.)

The first few slow turns of the wheel should produce flashes of light, from the point of view of the onlooker. The light goes through an opening between the slats of the wheel, hits the far mirror, and returns through the same opening. When the wheel speeds up, it will hit a certain speed at which the light disappears. The light will go out and hit the far mirror, but when it comes back it will hit the slat instead of the opening. The time it takes for the opening to give way to the slat equals the time it takes for the light to travel to the mirror and back. As the wheel speeds up even more, the light will reappear, because the wheel will be turning fast enough that the slat will have moved out of the way of the light. By calculating the time it takes for the wheel to move those two distances, the onlookers will know how long it took for light to travel to the mirror and back.

Fizeau was able to get a decent estimate of the speed of light by using his wheel. Later, it was refined to put an angled mirror, that also was rotated, behind the wheel. By looking at the exact angle at which the flashes of light hit the second mirror, scientists were able to estimate the speed of light as 298,000,000 meters per second.


via Science World.

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