MadSci Network: Physics
Query:

Re: speed of light experiments

Date: Tue May 19 17:05:45 1998
Posted By: Mike Francis, Other (pls. specify below), Physics/Astronomy, Self employed/ Amazing Discoveries Productions
Area of science: Physics
ID: 894998636.Ph
Message:

MadSci Network: Physics
Query:

Kelli,

     Back around 1600, Galileo is often credited with being the first scientist to try to determine the speed of light. His method was quite simple. He and an assistant each had lamps which could be covered and uncovered at will. Galileo would uncover his lamp, as soon as his assistant saw the light he would uncover his. By measuring the elapsed time until Galileo saw his assistant's light and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of the light. At the time he was teaching in Padua and spending much of his time in Venice. If he performed the experiment in Venice, using an island in the harbour for the student's lamp, he could concievabley have separated the lamps by over a mile. His conclusion, "If not instantaneous, it is extraordinarily rapid". Some accounts have him using his pulse to measure the time, a method I find unlikely due to its lack of accuracy. More likely he used a water clock, where the amount of water that empties from a container represents the amount of time that has passed. Even if he could have had the lamps 10 miles apart, the time he would have had to measure would be on the order of one millionth of a second.

    In 1675 the Danish astronomer Ole Roemer noticed that the times of eclipse for the moons of Jupiter seemed to depend on the relative positions of Jupiter and the Earth. If the Earth was close to Jupiter the orbits of her moons appeared to speed up, if the Earth was far from Jupiter they seemed to slow down. Reasoning that the moons orbital velocities should not be affected by their separtation the apparent change must be due to the extra time the light had to travel when the Earth was more distant from Jupiter. Using the commonly accepted value for the diameter of the Earth's orbit he came to the conclusion light must travel at 200,000 Km/sec.

    Hippolyte Louis Fizeau took a different approach to determining the speed of light in 1849. Fizeau used a light path about 5.36 miles long. Some of the light from a source was reflected off a half silvered mirror and some passed through it. The reflected light traveled 5.36 miles to another mirror, reflected back 5.36 miles, then it passed back through the half silvered mirror. An observer looking at the source through the mirror (the light source was focused using lenses so it would be distinct) would see the combined image from the original source and the reflected source. To time the light beam Fizeau used a spinning toothed wheel. When he got the wheel to spin at just the right speed none of the birsts of light that had traveled the 10.72 miles would make it back to the observer. By measuring the time it took for the wheel to turn from a tooth to a space and knowing how far the light traveled, Fizeau calculated a velocity for light of 313,300 Km/sec.

    Foucault improved upon the rotating wheel by using a rotating mirror. Light from a source was bounced off a rotating mirror to a stationary mirror. When the light returned to the rotating mirror it would reflect at a slightly different angle. The angle of the deflection could be used to determine the time the light had traveled and a speed determined. Michaelson refined this method over a period of more than 50 years to calculate a speed of light of 299,796 Km/s in 1926.


    You might be interested in a couple of diagrams on the speed of light at Reflection and Refraction .

  Mike Francis


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