## TomLikesPhysics 3 years ago I heard and read often that if one look´s at the spectrum of (say) the sun - you see that some colors in the spectrum are missing like for example the hydrogen line. This tell´s us that there is hydrogen in the sun. The hydrogen atom get´s energy - the electron get´s excited and jumps up one layer (or two?) (according to E=hf) and than jumps back and emits that energy. If the electron emits again exactly that energy E=hf which it "stole" earlier than why is there are color missing - shouldn´t the electron "donate" that color again if it sends the light out again?

1. shubham

In both the cases we have photons ( hf ) being absorbed or emitted. While discussing emission and absorption of photons by atoms we don't usually discuss where the photons go.

2. TuringTest

First off, the wavelength absorbed is not necessarily the wavelength emitted. A particle can absorb blue light and release it as infrared slowly (this is how objects heated by the sun emit light in the form of heat) Second, if the original wavelength photon is emitted, there is not guarantee which direction it will go. So absorption scatters the path of the light, hence we don't intercept it with out detectors.

3. TomLikesPhysics

1. So if you take a snapshot of the spectrum you do not look at the sun for a couple of minutes but only for a very short time-intervall? If I would long enough there should eventually light be emitted in my direction right I guess. 2. If the emitted light is not the f from E=hf which the electron got then the energy missing is transformed into heat than the electron enters a new orbit? Because now it can only jump a distance E=hf´with an f´<f.

4. TuringTest

1. If you are referring to an absorption spectrum, then the scattered light would be so dissipated by the time it reaches earth, the only or two photons per minute (at most I would think) from the source would probably be undetectable. Their paths are changing millions of kilometers from earth, so a deflection of the original photon by 1/2 a degree is more than enough to cause it to miss our detectors by thousands of kilometers. 2. I haven't really sorted that out myself, you should ask Jemurray3 or JamesJ, they're really knowledgeable about that stuff. I just knew my portion of the answer because I asked a physics teacher once the same thing. As far as what occurs within the atom, I think the electrons are stimulated to, say, 4 energy levels above the norm, then give off a photon as they descend each level (so that would be 4 longer wavelength photons) but that only can occur if the atom has its energy levels configured in a particular way; it would depend on the atom.

5. TomLikesPhysics

Ok, thank you all for your explanations. I was kind of confused but I think know I have a much better idea of what is going on with that kind of stuff.

6. TuringTest

You should probably note the difference between an emission spectrum and an absorption spectrum. If we view light directly from the sun we see the hydrogen line. This would be an emission spectrum. That's as opposed to light that, say, passes through a nebula which absorbs the hydrogen line (which means it must have hydrogen in it). In that case we would see an absence of that frequency, which is an absorption spectrum.

7. TuringTest

actually, sorry, the sun would produce a continuous spectrum a hot, rarefied gas would produce an emission spectrum...

8. TomLikesPhysics

The spectrum of the sun is continuous? I thought hydrogen and helium are missing. I think I even heard that helium was first "seen" on the sun is therefore called helium (from the greek or latin word for sun).

9. TuringTest

Helium was first detected in a spectral line that appeared during an eclipse, which I suppose allowed for a more detailed analysis than is possible directly analyzing the suns rays at full force. If the sun did not have a continuous spectrum it would basically only give off the spectral lines of hydrogen and helium (the heavier elements would probably not contribute enough light to notice easily) Light would not be white if not for the presence of an almost continuous spectrum. Light just consisting of the wavelengths of the spectral lines of hydrogen and helium would be a blue-green color I think, and would certainly have no ultraviolet frequencies.

10. TuringTest

I think they were able to detect the hydrogen because it was excited farther from the surface of the sun, and therefor not corrupted by the full spectrum of the continuous spectrum whilst they were blocked by the eclipse.

11. TuringTest

in case you doubt the almost continuous spectrum of the sun: http://en.wikipedia.org/wiki/Sunlight

12. TomLikesPhysics

Ah ok you meant almost continuous. I imagined it would not be continuous because there ought to be some gaps due to the absorption. http://www.saburchill.com/HOS/astronomy/images/181105001.jpg Just like there. But I guess now we are on the same page :)

13. TuringTest

Yeah, sorry if I wasn't clear about some things... I'm pretty the absorption spectra come from the fact that there is a layer of cooler hydrogen around the sun. The light emanating from the suns nuclear reactions is, I believe, continuous; but the hydrogen and helium lines are reabsorbed by the cooler gas at some distance from the surface. Not really my area of expertise though...