# Armand Fizeau's Speed of Light Experiment

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## 11 Questions

### What was the purpose of the spinning toothed wheel in Fizeau's experiment?

To modulate the light passing between its teeth for measurement

### What modification did Foucault make to Fizeau's experiment?

Replaced the toothed wheel with a rotating mirror

### How did Foucault determine that the speed of light was reduced in water compared to air?

By inserting a tube filled with water between the rotating mirror and a distant mirror

### What was the significance of Foucault's speed of light measurement?

It was much closer to today's accepted value of the speed of light

### Which theory did Foucault's experiment help disprove regarding the speed of light in water?

The corpuscular theory

### What aspect of Fizeau's measurements limited his calculation of the speed of light?

The precision of his measurements

### Who made the most precise measurement of the speed of light in the 19th century?

Albert A. Michelson

299,793 km/s

### Which method was used by Fizeau for the first terrestrial measurement of the speed of light?

Projection of light onto a distant mirror

299,979 km/s

Maxwell

## Study Notes

### Speed of Light Measurement

• In 1849, French scientist Armand Fizeau made the first terrestrial measurement of the speed of light.
• Fizeau's experiment involved focusing light through a beamsplitter onto an image plane with a spinning toothed wheel, and then projecting the light to a mirror 8 km away and back to the origin.
• By increasing the wheel's rotational speed until the light was blocked by a tooth, Fizeau calculated the speed of light to be 315,000 km/s, limited by the precision of his measurements.

### Improvements to Fizeau's Experiment

• French physicist Jean Léon Foucault modified Fizeau's experiment by replacing the toothed wheel with a rotating mirror.
• Foucault measured the speed of light to be 298,000 km/s, closer to the current accepted value.
• Foucault also demonstrated that the speed of light is reduced when traveling through water compared to air, disproving the corpuscular theory.

### Further Improvements and Modern Measurements

• Albert A. Michelson made the most precise measurement of the speed of light, averaging 299,774 km/s over many measurements.
• Modern technology has led to an accepted value of 299,793 km/s for the speed of light.
• Electromagnetic theory predicts the velocity of electromagnetic waves in free space to be 299,979 km/s, within 0.1% of the most precise measured values.

Learn about the first terrestrial measurement of the speed of light conducted by French scientist Armand Fizeau in 1849. The experiment involved a light source, a spinning toothed wheel, a mirror placed 8 km away, and beamsplitter to measure the speed of light.

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