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

What is the currently accepted value for the speed of light?

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

What is the value predicted by electromagnetic theory for the velocity of electromagnetic waves in free space?

299,979 km/s

Which scientist predicted the value within 0.1% accuracy of the precise measured values?

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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