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From the time of the Greeks it was thought that light had an infinite speed. In his 1638 publication "Two New Sciences", Galileo Galilei first suggested that the speed of light may in fact be finite but too quick for humans to measure. Nevertheless, he proposed an experiment whereby two people with lanterns stand a great distance from one another. One person uncovers their lantern and the other person uncovers theirs when they see the light from the first lantern. Of course, trying to time how long it took for the second person to uncover their lantern was impossible in the 17th century. In 1676, however, Olaf Roemer found by timing the eclipses of Jupiter by its moon, Io, that the speed of light was finite with a velocity of around 214000 km/s. Subsequent experiments boosted the speed up to around 300,000 km/s or 3 x 10^8 m/s. In 1873 James Clerk Maxwell unified the laws of electricity and magnetism, producing Maxwell's equations of electromagnetism. Maxwell found that the electric and magnetic fields traveled in the form of waves, with the electric field oscillating perpendicular to the magnetic field. The equations state that the velocity of propogation of the electromagnetic field is the same as light - c = 3 x 10^8 m/s. Hence, he concluded, light must be a form of electromagnetism. We are familiar with water waves which travel through water and sound waves which travel through air. Likewise, physicists believed that electromagnetic waves traveled through some medium which they called "aether" or "ether". To confirm the presence of the ether, physicists needed to show that the velocity of light varied slightly as the Earth moved through it, in the same way that a baseball thrown into the wind will travel more slowly than a baseball thrown with the wind. Measuring this difference in light speed was the purpose of the Michelson-Morley experiment, conducted in 1887. Albert Michelson and Edward Morley found, however, that the velocity of light did not change at all. A variety of subsequent experiments confirmed that the speed of light did not depend on the motion of the object that emitted the light. This was a very confusing result. For instance, suppose you are observing a train go by and there is someone inside the train who turns on a torch. One would expect that light shining in a forward direction from a moving train would appear to be travelling faster than light shining sideways from the train by an amount equal to the speed of the train. But the Michelson-Morley experiments demonstrated that the speed of light from the torch does not change, no matter which direction it is pointed. |
The speed of light has a (link popup window: http://www.ldolphin.org/chistory.html) *long history*.
Click (link popup window: http://www.what-is-the-speed-of-light.com/) *here* to learn more about other experiments to measure the speed of light.
See here for (link popup window: http://dbhs.wvusd.k12.ca.us/Chem-History/Roemer-1677/Roemer-1.GIF) *Roemer's original letter* to the Royal Society.
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