The Speed of Light

Just from watching lightning and listening to thunder, you know that the speed of light is greater than the speed of sound. The speed of light is very large. The speed of light is the greatest speed in the Universe.

c = 3 x 108 m/s

(that is about 186,000 miles per second!)

This means that it takes light only about eight minutes to travel the 140 million kilometers (or 93 million miles) from our Sun to Earth.

Galileo made attempts at measuring the speed of light by trying to measure the time required for light to travel from his lantern on one hillside to an assistant who then opened a shutter on his lantern on a distant hilltop. Galileo realized that he and his assistant were only measuring their own reaction times.

In 1675, the Danish astronomer Olaus Roemer found a discrepancy of about twenty minutes in the time that Jupiter's moons were eclipsed as they went behind Jupiter, depending upon Earth's position in its orbit around our Sun. This difference in time was due to the additional distance the light from Jupiter's moons had to travel at different times. Roemer used this information to calculate the speed of light.

In 1727 the British astronomer James Bradley calculated the speed of light using careful measurements of the change in a star's position . Bradley's measured value was very close to today's value of 300,000 km/s.

In the nineteenth century the French Physicist Hippolyte Fizeau measured the speed of light using a toothed wheel and a distant mirror. The speed of light could be determined by measuring the distance between wheel and mirror and by measuring the speed of the wheel.

A A Michelson, the Master of Light

A variation of Fizeau's experiment was carefully refined by the American Physicist Albert A Michelson.

Michelson had been a midshipman at the U S Naval Academy and then returned as an instructor. He carried out research there by measuring the speed of light with great precision. Today a line of round, brass markers show the location of the light beam in Michelson's early experiments. Michelson's apparatus is sketched below.

Michelson later moved to Case Institute of Technology in Cleveland OH where he and Edward Williams Morley from the Western Reserve University did further experiments involving light . These are known as the Michelson-Morley experiments and were quite important in the establishment of Relativity and Modern Physics.

We have already talked about the speed of sound. It requires about three seconds for sound to travel a kilometer and you may this to measure the time between seeing a lightning bold and hearing the thunder.

Q: How much time does it take for light to travel one kilometer?

A: For this, we can go back to the ideas we developed about motion. The speed of anything is the distance it covers divided by the time, v = d/t. That means the time required is the distance divided by the speed, t = d/v. With d = 1 km and v = 300,000 km/s, this means

t = d / v

t = ( 1 km ) / ( 300,000 km / s )

t = ( 1 / 300,000 ) s

t = 3.33 x 10-6 s

or, we may write this as the time is 3.33 microseconds.

Nature of Light


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(C) 2003, Doug Davis; all rights reserved