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 Originally Posted by MadMojoMonkey
The reason for saying "speed of light in vacuum" and not just "speed of light" is because light travels slower as it passes through materials based on the material's index of refraction, a material property. For example, light travels more slowly through glass than through empty space.
A farmer is having problems with his chickens. They are all suddenly getting very sick and he doesn't know what is wrong with them.
After trying all conventional means, he calls a biologist, a chemist, and a physicist to see if they can figure out what is wrong.
The biologist looks at the chickens, examines them a bit, and says he has no clue what could be wrong with them.
Then the chemist takes some tests and makes some measurements, but he can't come to any conclusions either.
So the physicist tries. He stands there and looks at the chickens for a long time without touching them or anything. All of the sudden, he starts scribbling away in a notebook.
After several gruesome calculations, he exclaims, "I've got it! But it only works for spherical chickens in a vacuum."
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