what is the cause of reflection?

A Dhingra

simply speaking ... out of phase means that the waves different phases ( or angle of phase shift is other than 0 degree).... is that right?

DaleSpam

If there is a vibrational mode of the right energy available then the photon will be absorbed instead of scattered. This is the principle behind infrared spectroscopy since the vibrational modes fall in the infrared energy range.

http://en.wikipedia.org/wiki/Infrared_spectroscopy

sophiecentaur

Simply speaking, waves with different phases have peaks and troughs at different times. You could 'see' this if you were to take a picture of the two waves - the peaks and troughs would appear in different places. Otherwise you could look at their variations in time (such as with an oscilloscope), in which case the peaks and troughs would be seen, also, to occur at different times.
Two signals (not necessarily waves) of the same frequency can also have a 'phase difference' between them. Two kids on swings of equal lengths may be going at the same rate but not coincide in time / phase.

The use of vectors (or, rather, Phasors) is a good way to show their phase relationship by displaying them as a 'frozen' circular motion. This method can show their amplitudes and phases and make it easy to show the result of adding the oscillations together. Wikkers is bound to have an entry on Phasors.

cesiumfrog

For someone who is interested in how reflection works on the microscopic scale, but doesn't have a strong technical background yet, I think Feynman's little popular science book "QED" is indeed the best possible resource.

It captures the excitement of the science, and it conveys the true technical mechanism without misleadingly oversimplifying and without needing the technical jargon nor confusing math.

(Phasors, on the other hand, are such an abstract and esoteric topic among physics graduates nowadays, hardly the first concept I would recommend someone begin from..)

cragar

So are you guys saying that reflection has to do with light having an E and B field . And it gets affected by the electrons in the material. What I always had trouble with is that light doesn't have charge so why would this affect the photon. And when I shine a laser into a B field it doesn't alter its path. I am probably missing something fundamental here.

DaleSpam

There is more to EM than just the Lorentz force law. EM fields interact with the fields, not just with charges (see Maxwell's equations). Otherwise you couldn't have waves in vacuum.

Born2bwire

The problem is that you, and this seems to be common, are mixing in both classical and quantum ideas. Thinking about charges and currents and using that as an explanation for the classical field reflection is perfectly fine. The incident fields induce currents on the surface and interior of the material via the Lorentz force. These induced currents create secondary fields which cancel out the transmitted fields and produce the reflected fields. Thus, the total field is an incident and reflected field without any transmitted wave (assuming total reflection). In terms of photons however, we need to look at the energy modes of the material and the band structure all of which are determined using quantum mechanics. It then can become a problem of scattering (in the case of reflection) and/or of transmission into one of the supported modes of the problem (where the diffraction can be solved for example via conservation of momentum). One can also use the idea of a path integral to get the same results (whose classical analogue is Fermat's principle).

sophiecentaur

You have apoint there, of course, but reflection, described as a wave phenomenon, requires a certain 'base level' of understanding. The idea that one should need to look at a book with Quantum Electrodynamics in the title suggests that it's even harder than it is.

It seems to be the fashion to leap into fundamental particles as an initial explanation of everything when a few very basic classical ideas can relate easily to concrete reality. I would say that any Physics graduate who struggled with the idea of expressing the amplitude and phase of a wave as a vector (just polar coordinates, surely?) would certainly be struggling with 'true understanding' of anything on the topic of fundamental particles.

Phasors, "esoteric"??? I have talked to may 'technician grade' workers who can deal more than adequately with the phasor in power engineering. Look at the threads on power factor on this forum. They're taken as read.

cragar

thanks dalespam and born2bwire for your answers.

A Dhingra

i think by now i have a rough idea as to what causes reflection.....
to just make sure what i have got is correct i am giving this reply....
for a mirror , an em wave moves through the surface of glass ( by scattering if the energy is less than that required for transition...)and at the surface of the metal ...the wave induces current ( here the incoming wave is absorbed in the conductor...) ..and thus gives out another em wave of same property as the existing one... making the angle of incidence equal to the angle of reflection....

is it correct by far?
if yes then only query left is that how do the angles become equal...... for 0 degree angle of incidence the em wave produced will move opposite to the entering direction....
what happens for other angles...