1. Time-of-Flight Methods:
* Rotating Mirror Method (Fizeau, Foucault): This method involves a beam of light reflected off a rotating mirror. The difference in the position of the reflected beam depending on the mirror's rotation speed allows calculating the speed of light.
* Michelson Interferometer: This setup uses interference patterns created by two beams of light travelling different paths. By measuring the shift in the interference pattern, the speed of light can be determined.
* Laser Pulses: Modern techniques use short laser pulses sent over a known distance. Measuring the time it takes for the pulse to travel and return allows for precise calculation of the speed of light.
2. Resonant Cavity Methods:
* Cavity Ring-Down Spectroscopy (CRDS): This technique involves trapping light within a resonant cavity and measuring the time it takes for the light to decay. The speed of light can be calculated based on the cavity dimensions and the decay time.
3. Frequency-Based Methods:
* Frequency Combs: These are lasers that produce a series of precisely spaced frequencies. By measuring the frequency difference between two specific lines in the comb, the speed of light can be determined.
* Atomic Clocks: These incredibly precise clocks use the frequency of specific atomic transitions to measure time. The speed of light can be determined by comparing the frequency of light with the frequency of an atomic clock.
Current Value:
The most precise value for the speed of light in vacuum is 299,792,458 meters per second (m/s). This value is defined as a constant in the International System of Units (SI), meaning that the meter is now defined in terms of the speed of light and the second.
Conclusion:
The measurement of the speed of light has evolved significantly over time, with each method contributing to increasing precision. Today, the speed of light is a fundamental constant used in defining other physical quantities, demonstrating its importance in modern physics.
