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Related: x-ray interaction with atmosphere

I know that electromagnetic waves of particular frequencies reflect from the ionosphere. And the light (which from one perspective is an electromagnetic wave) also reflects from the.. let's say snow. These types of reflection have the same nature or different mechanisms are involved here?

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Light is an EM wave from all perspectives, not just one. –  Manishearth Feb 13 '12 at 11:06
    
It's also particles (phatons), innit? –  ctapobep Feb 13 '12 at 11:37
    
All EM waves are photons. It's not just light that has particle nature, all EM waves have that nature. OK, I see what you're saying. I though you were segregating light from the rest of the EM spectrum from some esoteric perspective. –  Manishearth Feb 13 '12 at 12:07
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3 Answers

Electromagnetic radiation will reflect from any change in refractive index.

For light reflecting off snow you have an air/ice boundary, and the refractive index of air is about 1.0 while ice is about 1.3, so you get reflection. Actually you get multiple reflections which is why you get a diffuse scattering and why ice looks white.

For RF waves reflecting off the ionosphere, the ionosphere refractive index is changed by the free elections in it, so there is a refractive index mismatch between the very low pressure air below the ionosphere and the ionosphere itself and this causes reflection. It's a bit hand-waving to claim it's just that the ionosphere has a different refractive index, but that's a good starting point. If you want to look in more detail see http://ecjones.org/physics.html and many other easily Googlable articles.

So I suppose the mechanisms are basically the same.

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"ionosphere has a different refractive index" is correct, not hand-waving. Higher conductivity means larger imaginary part of refractive index - link. –  Steve B Jul 27 '12 at 12:05
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It was such a simple question, but you guys gave such complicated answers:(

It is really not that hard to answer: in both cases, we can model the reflection as light (an electromagnetic wave) passing through a medium with changing index of refraction. In both cases, we can analyze it as waves or rays, but it is better to analyze both as waves. In both cases, waves are reflected when the index of refraction changes (as Rennie already pointed out). In that sense, it is the same mechanism.

Now true, technically, what happens in the ionosphere is a bit more complicated, with radio waves being bent gradually back to earth rather than reflected at a sharply defined boundary with a discontinuity in index of refraction, but that is ignored when actual users of such radio communication (such as radio amateurs) work in terms of the (virtual) height of the relevant layer of the ionosphere (layers D through F2). See http://www.electronics-radio.com/articles/radio/basic_radio/propagation/ionospheric-hf-propagation.php for a simplified explanation of ionospheric reflection, http://www.qsl.net/zl1bpu/IONO/iono101.htm for more details.

Oh, there is one other aspect in which they are different: since ice and air are both dielectrics with ice having the higher index, the reflection is phase inverting. We don't usually care, because the phase is lost in the diffuse reflections anyway.

But since air in the ionosphere is conducting, we have to model it as a reflection where one medium is dielectric and the other conducting, so the treatment of phase is different. If it were a good conductor, the reflection would be phase preserving; but since the ionosphere is rather weakly conducting, the full electromagnetic wave treatment of this case is rather difficult, and is not found in elementary E&M texts: one has to resort to odd or old sources like Frankel to see it fully analyzed. Yet as the QSL site reference above mentions, one can at times hear the effects of phase distortion in HF signals reflected off the ionosphere. So the difference is relevant.

Come to think of it, my own answer turned out to be more complicated than I expected when staring it, too;) But I hope it has answered your question more fully.

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Probably to understand this I need to understand what refraction index is from the perspective of EM wave. Everywhere it's just described 'light goes through medium blah-blah', but it's not described why/how this medium changes light as EM wave on the level of particles/fields. It looks to be described here: farside.ph.utexas.edu/teaching/315/Waves/node39.html But need to re-read that several times to understand at least something :) –  ctapobep Aug 2 '12 at 14:02
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First of all, in order to describe this kind of phenomenon the EM "perspective" is enough, but if you want to look at the interaction between light and one atom you need the description given by quantum electrodynamics. So light is not an EM wave from all perspectives!

The reflection due to the ionosphere and the one you observe with snow are the same and it is called diffusive reflection. Because snow and ionosphere are not ordered/regular medium in fact you observe a reflection in all directions. With a mirror, for which atoms are regularly spaced, reflection appears only in one direction which explain Snell-Descartes.

I hope my answer is clear!

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But.. EM wave reflects from ionosphere not in all directions. E.g. at this page you can see k'' wave that is mirrored by the plasma:ecjones.org/physics.html which is used for radio waves propagation actually. –  ctapobep Feb 13 '12 at 11:44
    
So light is not an EM wave from all perspectives!?? Why so?? –  Vineet Menon Feb 13 '12 at 15:46
    
The right description of what is called EM wave since Maxwell is given by Quantum Electrodynamics (read The Strange Theory of Light and Matter, Feynman). As you maybe know, a great debate during 17th century was about the nature of light : is it a wave or made of particles? Newton thought that light it is composed of particles in order to explain reflection ; and Huygens thought that it is a wave. Indeed you can't explain interferences with classical particles. –  PanAkry Feb 13 '12 at 16:25
    
During the 19th century the theory exposed by Maxwell confirmed that light is a wave and all phenomena were explained with this theory. At the beginning of the 20th century, Planck and Einstein explained the black body and the photoelectric effect only with help of photons, which contradict Maxwell's theory! Then the complete theory called Quantum Electrodynamics was achieved by Feynman, Schwinger and Tomonaga. Nowadays this theory explains all phenomena which involve matter and light (Lamb shift, stimulated emission), which is not the case of Maxwell theory. –  PanAkry Feb 13 '12 at 16:38
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