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Q & A: light fading

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Most recent answer: 10/22/2007
Q:
suppose a white light is passed in space, after certain distance it dissapears. my question is whether the light loses its energy or whether the dark absorbs the energy.If it loses what is the energy form its changed into?
- lourde (age 25)
madras veterinary college, chennai- India
A:
The main effect you may be thinking of is just that the light spreads out. As it does, its intensity in any one place goes down, keeping total energy constant.

On a much longer time scale, effects of General Relativistic cosmology become important. The space the light is in is expanding, diluting its energy further. The wavelengths stretch out too, so there is also less energy per photon. That's why the background radiation from the Big Bang no longer looks like light from something like the surface of the Sun but rather consists of microwaves. In this process the light energy density goes down faster than the volume goes up, so it looks like energy is being lost. Howver, I believe that the resolution is that a gravitational potential energy term goes up, keeping the total amount of energy constant.

There can also be simple absorption by dust, which converts the energy to thermal vibrations in the dust. Some is then re-radiated as lower frequency radiation. This process does not seem to have been very important on a cosmological scale.

Mike W.

(published on 10/22/2007)

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