[CsManiacDan (OP)]

I read a book recently that was all about the theory of Quantum Electrodynamics. This strange theory that can explain how photons and electrons interact. As a starting point for trying to explain how it works, the book pointed out the phenomenon of partial reflection. that is, when some photons pass straight through glass and others are bounced straight back off it.

The book stated and explained that between 4% and 16% of the photons meeting the glass bounce back off it (depending on the thickness of the glass.) I understand why this is but what i would like to know is how is it that this percentage of photons 'know' that they are supposed to bounce back up.

I find this topic quite tricky at the best of times, so any help would be very welcome.

I Love Caesium!!!

[tweener]

You are definitely not the only one that finds this topic tricky!

----
John - The Eternal Pessimist.

[qpan]

I don't think that the photons "know". When photons pass near electron shells, if they have the exact amount of energy required to "promote" an electron to a higher energy shell, they will be absorbed by the electron. However, a short time after, the electron falls back to its original energy state and a photon is emitted, which may go in any direction. (this is of course a classical model).

Quantum theory dictates that a particle travels all possible routes to a destination, and that the route is not know unless the particle is measured, when it will pop out of its quantum superposition and "decide" a path. The path is not really decided, but rather it is down to chance. So i think it is down to chance rather than the photon's choice. As when the photon interacts with matter, it is the equivalent of it being measured and so, the quantum superposition crystallises into a single path.


"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

[Dan B]

Consider the difference between a "X-ray" photon and a "light" photon. [] Now consider the source of the photons - it probably doesn't constantly emit photons at exaclty the same wavelength...

[tweener]

Other than energy (i.e. wavelength) there is no difference between X-ray and "light" photons.  A source of photons is too general.  If you are talking about a certain type of source, such as a hot hydrogen plasma, under low pressure, the emissions will not all be at the same wavelength, but they will be quantized into certain wavelengths that are characteristic of the hydrogen atom.  If you have a comglomeration of different materials all stuck together, you will get a wide spread of wavelengths because of different types of atoms and molecules and the interaction between them.  If you consider a laser, it will always produce photons of one certain wavelength (depending on the type of laser), and moreover, they will all be in phase (coherent).

And, welcome to the forum Dan!

----
John - The Eternal Pessimist.

[Dan B]

Thanks for the welcome []

I was just (badly) saying that you would classically expect some of the photons to be "reflected." We have been using things that partially reflect for a while in graviational wave detectors. They are very useful.

But as far as quantum reflection goes, from what I understand, it comes from QM allowing negative kinetic energies, so there is always an imaginary part to a wave in QM. When the imaginary wave component passes into a new medium its wavelength changes, if the new wavelength "fits" into the medium depth there is no relfection.