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Asked by Hannah to Greg, Laura, Mobeen, Paul on 16 Mar 2016.
Question: Is the speed of light faster in a vacuum?
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Paul O'Mahoney answered on 16 Mar 2016:
The ‘speed of light’ is defined as, quite appropriately, the speed of light in a vacuum (this is just under 300,000,000 metres per second). And yes, light does in fact travel a bit slower if it has to pass through something, like air or water or glass. This is what the refractive index of materials can measure, the ratio of the speed of light in a vacuum to the speed of light in the material.
When people say that nothing can travel faster than the speed of light, they really mean the speed of light in a vacuum.
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Greg Melia answered on 16 Mar 2016:
Paul has it.
Do you know how light ‘bends’ when it goes through water? That’s because light travels slower in water – because if a light beam is travelling at an angle to some water when it hits it, the first bit of the light to hit the water travels slower for longer.
The lenses in your eyes use this effect to ‘bend’ (refract) light to focus onto a point in your retina, where it hits the optic nerve and creates a signal that is transmitted to the brain. Spectacles use the same effect to refract the light a bit more if the lens in your eyes isn’t quite powerful enough. This works because light also travels slower in glass than in a vacuum.
Something really weird is called Cehrencov radiation, and happens in nuclear reactors. Nothing can travel faster than the speed of light in a vacuum, but a nuclear reaction can accelerate particles up to speeds that are faster than the speed of light in the reactor material, (which is probably water). Have you ever heard a sonic boom when an aircraft travels faster than the speed of sound? That’s what happens here: the water emits an electromagnetic shock wave, which you see as an eerie blue glow.
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