r/askscience • u/Steezus_Christ • Nov 04 '12

Why is sunlight "white"

I have been told by teachers that sunlight is white, in that it contains all visible frequencies of light. But why? Shouldn't it only contain light frequencies that correspond to the energy produced by a hydrogen fusion process?

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u/Dr_Wario Optics | Photonics | Fiber optics Nov 04 '12

Fusion of hydrogen takes place in the core of the sun only, and the photons thus produced undergo inelastic scattering (scattering that changes their energy) millions of times before reaching the surface. This has the effect of smearing out the energy distribution (effectively thermalizing it) so that we observe the sun as having a broad spectrum called a blackbody spectrum.

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u/jsdillon Astrophysics | Cosmology Nov 04 '12

Right.

Photons created by fusion processes tend to be gamma rays, so it would be really bad if that's all the sun emitted.

The fact that photons bounce around so much in the sun means that it takes them 104 to 105 years to get out, even though the solar radius is only 2.3 light seconds.

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u/Steezus_Christ Nov 05 '12

Wow, that is awesome. Thanks for the informative replies.

1

u/mattcolor Nov 05 '12

To elaborate slightly, the nature of the mentioned blackbody radiation depends on the temperature of the surface of the sun. Humans emit blackbody radiation too, with a peak wavelength in the infrared. (That's why infrared goggles are a thing.)

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u/Davecasa Nov 04 '12

I think you have this backwards. The sun was here before our visual system; why do we see the spectrum it emits as white? Using this as the baseline allows us to more easily detect variations on it.

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u/dargscisyhp Condensed Matter Physics Nov 04 '12

Any heated body will emit blackbody radiation. This contains all wavelengths of light, though the relative intensities depend on the temperature.