# Does the efficiency of radiation change in any way in the presence of a medium?

I've read that radiation doesn't really require a medium. But if you're taking, for example, the sun's light, then does its efficiency of transmission increase or decrease once it reaches the earth's atmosphere? (Even though the speed of light is taken as a constant, does that necessarily mean the presence or absence of medium is irrelevant to it's speed?)

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What exactly do you mean by "efficiency?" – Colin K Mar 30 '12 at 16:57
i mean the speed. I've already mentioned the confusion I'm having about the speed of light... it's taken as a constant, so does that mean the presence of medium (air) changes nothing? – sciencerocks Mar 30 '12 at 17:03

The constant called "The speed of light" is really more correctly called "The speed of light in a vacuum." In any material medium the light will move slower. The amount by which it moves slower is depends on the medium, and is quantified by that mediums "index of refraction." The index is usually denoted $n$, and the speed of light in a medium is $$c' = \frac{c}{n}$$

$n$ can also depend on the wavelength of the light. This wavelength dependance is called dispersion, and is the reason for chromatic aberration in a lens.

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thanks! its what i needed. – sciencerocks Mar 30 '12 at 17:20

Let me suggest a slightly a different perspective:

If by "the efficiency of radiation" we mean the rate of spontaneous emission, then yes, the environment can alter it significantly --- consider example of an atom in a cavity or the supperradiance effect.

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## protected by Qmechanic♦Mar 3 '13 at 18:57

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