Does light wear out?

12 January 2016

Question

One question asked in my primary science class, when learning about energy, is 'when does light wear out?' and 'how far does light travel?' and I am unsure how to accurately. Can you help?

Answer

Kat Arney put this question to physicist Stuart Higgins...

Stuart - The short answer is - no it doesn't wear out. It never wears out, it keeps travelling for ever and that's because light actually is a wave of energy, it doesn't have mass, and so things that have mass, normally decay into smaller things and break down. In this case light doesn't have it. It will just keep going for ever. However, that doesn't really match our everyday experience. If I shine a torch in your eyes and it's blindingly bright, and then I stand the other side of a field and shine it towards you, it looks dimmer, it doesn't seem as bright, so what's going on there? There's different things that actually that could make light wear out as it were, and that's where the light might be absorbed by the particles of the air, or the atmosphere, or scattered off the dust. Or it might be the lights just not reaching you, it's shooting at a slightly different angle.

Kat - So, if there was a perfect vacuum in the field between you with the torch and me, then it would seem as bright as if I was standing right next to you.

Stuart - Not quite. So if it were maybe a laser, and the laser was shining and the laser travelling in a straight line, that would be fine but, actually, the torch, if you think where the lights being made by the filament, it passes through a lens, that scatters some of the light, it passes through. There's usually a bit of metallic foil in their as well that kind of pushes it toward the front of the torch, but even still the light is spreading out and so, if you imagine the number of photons that are reaching your eye, it's actually decreasing the further away you are as your eye is only a fixed area that can receive them.

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