The Speed of Light

The Speed of Light



The Speed of Light


Though it may not seem like it, light takes time to get from one place to another. The light we see from the stars at night was emitted from the star a very long time ago. In fact, the star might have burned out by the time its rays reach us, but we can't see it because it takes times to reach our solar system.

The first clue towards the speed of light came from outer space, from the planet Jupiter. In 1676, Danish astronomer Olaus Roemer observed the difference in peroids that Jupiter's moons had to the Earth's orbit. He realized that the light reflected off the moons had a different amount of light to travel depending on the Earth's position. Using this knowledge, he used mathematics to estimate that the speed of light was 136,646 miles per second. Unfortunately, Roemer didn't know that his calculations were too low, but his work was a large accomplishment for his time.

Armand Fizeau made the first land-based estimate in 1849. Fizeau set up an experiment where he placed a light five and a half miles away from a mirror. In front of the light, he set up a wheel with toothed edges. When the light would shine, it would pass through the gap between two teeth. The mirror would reflect the beam back and would shine through the next gap between the teeth. Fizeau could then estimate the speed of light from the speed at which the toothed wheel was moving. He measured the speed at 194,410 miles per second.

A year later in 1850, Léon Foucault made some changes to Fizeau's experiment. Instead of using a toothed wheel, Foucault replaced it with a spinning mirror. The light would shine onto the spinning mirror and be reflected towards a series of stationary mirrors. By measuring the amount of distance the light had to travel and knowing how fast the mirror was spinning, Foucault figured the speed of light to be 185,093 miles per second. Foucault's results were very close to the actual speed of light.

Once lasers were invented, scientists were able to measure the speed of light much more easily. In 1972, the National Institute of Standards and Technology discovered that the actual speed of light was 299,792,458 meters per second, or 186,282 miles per second (just 189 miles different than Foucault's measurement). Because of these new results, the General Conference of Weights and Measures fixed the meter as the distance light travels in 1/299,792,458 of a second.

However, the speed of light does vary, depending on the medium. Albert Einstein proved that light is the fastest moving thing in a vacuum, with its speed being roughly 186,000 miles per second. In water, the speed of light slows to 140,000 miles per second. Its speed is 124,000 per second in glass. Through a diamond, the speed of light is 77,500 miles per second.


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