APOD: A Colorful Lunar Corona (2015 Jun 15)

[Chris Peterson]

I would call this something else...

I'd say treat "lunar corona" as a compound word, not the word "corona" modified by "lunar". I think the existing name is here to say, mildly confusing or not.

[Markus Schwarz]

I did some searching and found this helpful article about lunar coronae by Les Cowley et al.. Here I give a brief summary:

A lunar corona results from diffraction of moon light off water droplets. It is different from a rainbow in that the latter results from reflection, refraction, and dispersion in water.

Whether diffraction or refraction dominates depends on the ratio between the wavelength in question and the size of the droplet. Visible light has a wavelengths of about 500 nm. Raindrops have sizes of about 2mm, much larger than the wavelength of visible light, and reflection and refraction dominate (resulting in rainbows). The droplets causing the luna corona have a size of about 10micro meters, wich is only about a factor of 10 larger than the wavelength. Hence, diffraction plays a more dominant role.

There are different levels of rigour when it comes to diffraction calculations. As a first step (also mentioned by Neufer) on can use Frauenhofer far-field diffraction. In this limit, the diffraction pattern from a single droplet is the the same as that of a disk of the same size (Babinet’s theorem). The pattern is then given by the Airy disk.

When all drops have the same size, the resulting corona looks like today’s APOD. When the drops size has a narrow distribution the corona can be noncircular. An extreme case is iridescence.

A more general approach, that is needed when the drop size is “small”, is Mie scattering theory. Computer codes can be used to solve the equations and yield the correct corona properties. The article gives several examples.

(end of summary)

I haven’t found a reference to Mie theory yet, but from the above article, Mie theory is a solution of the classical Maxwell equations and does not involve QM in any way. It is a general Ansatz for (classical) scattering theory, but requires computers to solve effectively. If it successfully describes lunar coronae I don't understand why one would want to say that they are a QM phenomenon. But this does not mean that all scattering problems can be described by classical physics. As a rough estimate, I would say that classical scattering theory breaks down once the wavelength of light becomes comparable to the size of molecules and atoms.

[JohnD]

Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.

This is not a rainbow. It isn't caused by refraction, dispersion, and internal reflection, but by diffraction, a completely different process. So there's no reason to think the structure should be similar.

(If I have time later, and nobody jumps in first, I'll try to devise a simple explanation for how diffraction describes what we're seeing. It's not complicated, but it's not easy to put into words, either.)

Thank you, Chris! And subsequent answerers!
So is this more like the diffraction seen from oil on water? That's another place where successive colour 'rainbows' are seen.

http://hyperphysics.phy-astr.gsu.edu/hb ... lfilm.html

And refraction is not a quantum effect, while diffraction is?

John

[Markus Schwarz]

And refraction is not a quantum effect, while diffraction is?

No. Whether a phenomenon needs to be described by classical physics or QM depends on the scales of the problem. One such quantity is the de Broglie wavelength of the system. If the spatial extend of the system is comparable to it's de Broglie wavelength, QM must be used.

Both refraction and diffraction can occur in classical as well as quantum mechanics. A classical example is the diffraction of water waves, while a rainbow is a classical example for refraction of light rays. The double-slit experiment with electrons is an example of a quantum mechanical diffraction, which is caused by the wave-like nature of the electron.

However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

[Chris Peterson]

I haven’t found a reference to Mie theory yet, but from the above article, Mie theory is a solution of the classical Maxwell equations and does not involve QM in any way.

Depends what you mean by "Mie theory". This is true if you're discussing the theory developed by Mie, but Mie scattering is a physical phenomenon that can be treated by either classical or QM analyses.

[Chris Peterson]

However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.

[Chris Peterson]

So is this more like the diffraction seen from oil on water? That's another place where successive colour 'rainbows' are seen.

The colors we see on an oil film are caused by interference between reflections off the top and bottom surfaces of the film. Diffraction is not involved.

And refraction is not a quantum effect, while diffraction is?

Either can be analyzed by classical or quantum methods. Classical analysis tends to fall apart at small scales or for a small number of particles, quantum analysis tends to fall apart at large scales or a large number of particles. The phenomena are what they are, neither classical nor quantum. These concepts refer not to the phenomena themselves, but to how we can best model them.

[Markus Schwarz]

However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.

Chris, now you confused me. To clarify, I talked about classical particles (a tennis ball, for example). The de Broglie wavelength of the tennis ball is much smaller than the ball itself. This means that the wave nature of the tennis ball is practically unobservable. We agree on that? Where do you observe diffraction of classical particles?

On the other hand, an elementary particle, e.g. an electron, can display wave-like behaviour, like diffraction. In that case, the diffraction is described by QM, of course. This is probably best discussed in the framework of quantum field theory, which "unifies" particle and wave treatment.

But the transition from the quantum to the classical domain is not sharp and depends on the sensitivity of the detector and the isolation of the setup. Then it is possible to observe diffraction of even complex molecules.

[Chris Peterson]

However, diffraction is always a wave phenomenon. It does not occur with rays or classical particles.

No. Experimentally, diffraction occurs with particles (even a single particle) and can be treated by QM in both the wave and particle domain.

Chris, now you confused me. To clarify, I talked about classical particles (a tennis ball, for example). The de Broglie wavelength of the tennis ball is much smaller than the ball itself. This means that the wave nature of the tennis ball is practically unobservable. We agree on that? Where do you observe diffraction of classical particles?

As I noted elsewhere, a QM analysis generally fails for a large collection of particles. So we are in agreement that the wave nature of a tennis ball is effectively unobservable. But diffraction is observed for single photons and single electrons using carefully designed experiments. In this realm, the analysis is based on QM, on the description of a particle by its wavefunction, not by its classical parameters.

On the other hand, an elementary particle, e.g. an electron, can display wave-like behaviour, like diffraction. In that case, the diffraction is described by QM, of course. This is probably best discussed in the framework of quantum field theory, which "unifies" particle and wave treatment.

I don't think we're in any disagreement, we're just using slightly different terminology. I just wanted to clarify a possible misconception that calling something a wave phenomenon might be understood to mean that it demanded a classical treatment (not your misunderstanding, but a possible misreading in the context of the preceding discussion).

[DavidLeodis]

In the information brought up through the 'Strawberry Moon' link it mentions that Saturn and Antares were close to the Full Moon on June 2 2015, so I wonder if the obvious bright spot below the Moon is one of those (possibly Saturn) or just an image artefact.

[JohnD]

And now (tarantara!), an Earthbound Corona.

See the Earth Science Picture of the Day for today 17th June 2015 at http://epod.usra.edu/
Same effect, allegedly, except with pollen grains!

John

[Atomictom64]

I recognised the Moon as a Southern Hemisphere image. And, Bingo! The city of La Plata is within one minute of latitude of my city of Adelaide, South Australia. I love the southern moon with its "rabbit" image - I think that's an African legend. Viva el sur

[Chris Peterson]

I recognised the Moon as a Southern Hemisphere image. And, Bingo! The city of La Plata is within one minute of latitude of my city of Adelaide, South Australia. I love the southern moon with its "rabbit" image - I think that's an African legend. Viva el sur :)

You can't tell what hemisphere an image of the Moon is made from if you don't have a horizon for reference. And even so, you may not be able to tell, since the orientation of the Moon changes over the night and it depends on knowing which horizon you are using for reference. You see the same rabbit, in the same orientation, from the northern hemisphere, just not at the same time.