The speed of light has been measured through various ingenious experiments over time, evolving in precision with advances in technology. Here are some key methods:
1. Rømer’s Observation (1676)
Danish astronomer Ole Rømer was the first to estimate the speed of light. He noticed that the timing of Jupiter’s moon Io’s eclipses changed depending on Earth’s position in its orbit. He reasoned that light had a finite speed and took longer to reach Earth when it was farther from Jupiter.
2. Fizeau’s Toothed Wheel Method (1849)
French physicist Hippolyte Fizeau sent light through a rotating toothed wheel toward a distant mirror. The wheel’s rotation controlled whether light passed through a gap or hit a tooth on the way back. By adjusting the speed of rotation and knowing the distance, he calculated light’s speed.
3. Michelson’s Rotating Mirror (1879–1926)
Albert A. Michelson improved on Fizeau’s work using rotating mirrors. Light was reflected over long distances to a mirror and back. By adjusting the rotation rate of the mirror and measuring the shift in the reflected beam, he obtained a highly accurate value for the speed of light.
4. Modern Methods (Lasers and Electronics)
Today, scientists use lasers, ultrafast detectors, and precise timing circuits. A pulse of laser light is sent to a mirror and reflected back. The travel time is measured using atomic clocks or electronic timers. Given the distance and time, speed = distance/time.
Final Result:
With modern technology, the speed of light in a vacuum is now defined as exactly 299,792,458 meters per second, and the meter is defined based on this constant.