I read in my Physics textbook that when light falls perpendicularly on an optically denser medium it doesn't change its path rather it's speed slows down. If after multiple refractions the speed of light becomes equal to the speed of the object, what will happen? What if the speed of light was never $c$ and was equal to 700,000 km/h (the speed of Parker Solar Prove, a NASA spacecraft)? If it were 700,000 km/h, would it not defy the concept of an object not travelling with the speed of light? If you use some complex mathematics, please do explain them, because I might not be familiar with them.
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A massive object can't travel faster than c, the speed of light in vacuum, which is a universal constant.
An object can travel through some medium faster than the speed of light in that medium. When it does, it emits Cherenkov radiation — the characteristic blue glow of nuclear reactors — much as an object traveling faster than the speed of sound in a medium emits a sonic boom.
hobbs
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If after multiple refractions the speed of light becomes equal to the speed of the object, what will happen?
You don't need multiple refractions. This is a common scenario in nuclear reactors. They are very often placed in a pool of water, and the electrons that are ejected travel faster than the speed of light in the water. They produce a characteristic blue glow called Cherenkov radiation.
This does not violate relativity. The invariant speed $c$ is the speed of light in vacuum. It is the invariance of $c$ which leads to all of the relativistic effects, not the fact that light goes that speed in vacuum. If the photon were eventually found to have some small (very small) mass, then $c$ would not be the speed of light even in vacuum, but it would still be the invariant speed and all relativistic effects would hold.
Dale
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