Fizeau Experiment

Introduction

The Fizeau experiment, conducted by French physicist Armand Fizeau in 1851, was one of the first direct measurements of the speed of light. Fizeau’s method was ingenious and had a significant impact on the development of theories regarding the nature of light and its propagation.

Experimental Setup

Fizeau directed a beam of light at a mirror several kilometers away. Between the light source and the mirror, he placed a rapidly rotating cogwheel. When the wheel was at rest, the light beam could pass through one of the gaps in the wheel, bounce off the mirror, and return through the same gap.

As the wheel’s rotation speed increased, there was a specific speed at which the returning light beam would be blocked by a cog. By knowing the rotation speed of the wheel and the distance to the mirror, Fizeau was able to calculate the speed of light.

Key Equation of the Fizeau Experiment

The speed of light can be calculated using the formula

where:

• is the distance to the mirror,
• is the speed of rotation at which the returning light beam is blocked by a cog (in rotations per second), and
• is the number of cogs on the wheel.

Impact and Importance of the Fizeau Experiment

Fizeau’s measurement of the speed of light was remarkably accurate for its time, within 5% of the currently accepted value. This experiment provided one of the earliest confirmations of the wave theory of light, as the results were inconsistent with Newton’s corpuscular theory of light.

The experiment also presented evidence to rule out the theory of a “luminiferous aether,” a medium through which light was thought to propagate. This led to the development of the theory of special relativity by Albert Einstein.

Modern Variations and Improvements

Since Fizeau’s original experiment, more precise methods of measuring the speed of light have been developed. Nonetheless, the Fizeau experiment remains a classic illustration of an ingenious and elegant experimental method. In fact, a version of the Fizeau experiment using lasers and fast-moving streams of water has been used to confirm one of the predictions of the special theory of relativity, the Fresnel drag coefficient.

Conclusion

The Fizeau experiment represents an important milestone in the history of physics. Not only did it provide a novel method for measuring the speed of light, but it also contributed significantly to our understanding of the nature of light itself. Even today, it continues to inspire both experimental and theoretical investigations into the nature of light and its propagation.

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