here - Department of Physics, HKU
here - Department of Physics, HKU
here - Department of Physics, HKU
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CHAPTER 1. PRE-SPECIAL RELATIVITY PHYSICS 7<br />
Since light is a kind <strong>of</strong> wave, scientists before Einstein would ask what the<br />
medium <strong>of</strong> the light wave is. To them, the medium <strong>of</strong> water waves is water,<br />
the medium <strong>of</strong> sound waves is air and the medium <strong>of</strong> light waves is ether,<br />
although they did not have direct evidence <strong>of</strong> its existence yet. Then, the<br />
rest frame <strong>of</strong> ether is very important. We, <strong>of</strong> course, now know that ether<br />
does not exist, but many famous scientists at that time supported this idea.<br />
After the discovery <strong>of</strong> quantum electrodynamic in 1940s, we know that<br />
light also has some particle properties. The fundamental unit <strong>of</strong> light is called<br />
photon. We will only use the term, not the theory.
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