Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Diffraction, refraction & reflection

  1. Oct 18, 2003 #1
    I have a simple question to which, I assume, the answer will be quite complicated. I asked my physics teacher this question and all he would say was, "It is known."
    So without beating me to death with physics can anyone tell me why waves diffract? For instance, electron diffraction around a nucleus.

    I also would like to know why things refract. I have learnt how, when a photon comes into contact with an atom, electrons are promoted to higher energy levels if the photon is of a high enough energy. But this is the only way in which I know of how em-radiation can interact with matter.
    Once again I asked my physics teacher and all he would say is that, "In the prescence of certain magnetic fields, light can interact with matter," or words to that effect, (I can't remember exactly what he said, or even if it was magnetic fields that he refered to).

    Another little side-bar, if a ray of light hits a perspex block at less than the criticle angle, it enters the block and refracts. If a ray of light hits a perspex block at greater than the criticle angle, it reflects. But if a ray of light hits a perspex block AT the criticle angle, it travels along the edge of the block, but is the ray of light inside or outside the block? [?]
    Oh yeah, that's another one. Why does em-radiation reflect? I suppose this will be intimately related to refraction but it would be nice to know.
    Last edited: Oct 18, 2003
  2. jcsd
  3. Oct 18, 2003 #2


    User Avatar
    Science Advisor
    Gold Member

    Difraction is pretty easy to explain, it can easily be demostrated with water waves, basically when waves are out of phase and they meet (of course were assuming that they are coherent) the sum total of them at that point is zero (assduming for the sake of simplicty that they have the same amplitude a frequency), and when they are in phase the sum total of the two is twice the orginal amplitude.

    IIRC refraction is caused by virtual interactions with the material, which though being virtual still take time and deflect thus cause the light to 'slow' which is what is classically responsible for refraction.

    Phtons can interact with electrons by simply colliding with them and being defelcted like a classical particle in a process called Compton scattering.

    Refelction is quite simple really as it is just the absorbtion and re-emmission of phtons by electrons on the substance surface.
  4. Oct 18, 2003 #3
    There are just a few things that I don't quite understand about that post, but if you would be king enough to clear them up for me I'm sure that I will fully understand.

    For simplicity, I'll bulletpoint them:

    * I'm not sure what you mean by coherant, "of course were assuming that they are coherent."

    * I've never even heard of IIRC refraction or virtual interactions. Also I generally think of refraction as a car mounting the pavement at an angle, where one side of the car slows down before the other side, therefore causing it to turn (or refract). I suppose this depends on what a virtual interaction is but, what is causing the light to turn (or refract) here?

    * If reflection is just the absorbtion and re-emission of photons, why is it that the angle of incidence always equals the angle of reflection? Surely if they are re-emitted they will be re-emitted in a random direction.

    Do you have any thoughts on that critical angle thing by the way?
    I'm young, dumb and hungry for physics knowledge. (I know it's lame but it sounded good in my head) :smile:.
  5. Oct 18, 2003 #4


    User Avatar
    Science Advisor
    Gold Member

    *coherent means that there is a fixed phase relationship between corresponding points on the wave, or simply the waves are not all 'jumbled' up but are arranged in a consitent pattern or the light is in phase with itself.

    * IIRC, stands for "If I recall correctly". Interactions in static fields are mediated by virtual particles, So in non-quantum terms it's just the interaction with the static electromagnetic field. virtual particles have the property of only 'sort of' existing.

    * I should of been clearer I was referring to defuse refelction which is what we generally see when light hits an object, in specular reflection such as takes place in a mirror the photns are not absorbed but 'bounce' off the surface without directly interactiong with it.
  6. Oct 18, 2003 #5
    Thanks for clearing those points up for me. That's another gap in my knowledge filled in.

    By virtual particles, I assume you mean things like gauge bosons such as gluons and W+ boson.
    They were explained to us (the class) as coming into existance for an infinitesimely short time by "borrowing" energy from the universe to mediate interactions between particles.

    Bo-selecta, (I tell thee).
    Last edited: Oct 18, 2003
  7. Oct 18, 2003 #6


    User Avatar
    Science Advisor
    Gold Member

    Yes I was referring to a virtual gauge boson- the virtual photon. Gluons and W+ bosons may be real particles too and virtual particles aren't limited to just the gauge bosons but can be any species of particle.
  8. Oct 18, 2003 #7
    Well so far we've only done about gauge bosons so I don't know about any others yet. Unfortunately I don't think that we're going to do any more quantum mechanics and I'm going on to do aeronautics at University next year, so I guess I've reached a glass ceiling there.

    Anyway, thanks for the info.
  9. Oct 18, 2003 #8
    All matter has wave and particle properties. In general, the smaller the scale, the more prominent the wave properties. When two masses approach each other on this scale, their interaction tends to take place as wave interference. One consequence of this interference is action that takes place diminishingly with increasing distance from the region of maximum wave amplitude. Unlike particle interaction, wave interaction takes place, albeit negligibly, at a distance approaching infinity. Also, particles like electrons may behave as waves with a de Broglie wavelength [lamb].

    You may know of the Heisenberg uncertainty principle, where h [<=] [lamb]p=A. This states that real actions (A) must be greater than Planck's constant (h), in other words, that the wavelength ([lamb]) and the momentum of a quantum wave packet (wave-particle) vary inversely at and above the product h. The smaller the quantum's wavelength, in general, the greater its momentum is, and vice versa.

    Looking closely at a razor's edge, you can see constructive and destructive interference. Since the light is not completely incoherent, light and dark bands will appear where photons rarify and condense. Young made a similar experiment where, as you know, a wall with two slits was illuminated from one side to construct an interference pattern at a screen opposite the other. Feynman considered this to be the epitome for quantum interpretation. In its many versions it shows that wave properties are nonlocal, that is, influence superluminally other quantum events, but in a probabilistic manner.

    Diffraction demonstrates the microscopic statistical structure of matter by a "quantum wavefunction" much the same way as water waves determine a field of amplitudes and phases. The wavefunction assumes actions discrete in units of h, though. It may be thought of a "probability wave" that determines the magnitude of physical properties ("observables"). These may include momentum or wavelength, as the argument of the wave property is fundamentally action, A=[lamb]p [=>] h.
  10. Oct 18, 2003 #9
    As it happens I am familiar with deBroglie wavelengths and Planck's constant, however, I am not familiar with Heisenberg's uncertainty principle.

    E=pc where E=fh,
    f=pc/h where also f=c/[lamb]
    therefore [lamb]=h/p but h is a constant

    therefore [lamb] [pro] 1/p

    I know that the wavelength of a photon (or anything else for that matter) is inversely proportional to it's momentum. Although I'm not sure why this would link in with an uncertainty principle, it seems pretty predictable to me.

    I know a little bit about diffraction pattens and diffraction gratings, (we touched on them briefly).

    As far as Young is concerned, I only know about the Young modulus (how stretchy wire is), although I don't know if it is the same Young.

    I always thought that particles and waves would only interact with other particles and waves upon very close approach, such as the strong nuclear force in the nucleaus of an atom.

    Well after reading through what you've written, I understand about 30% of it. Maybe after a little more scratching around and chipping away at the teacher I can bump it up to 50.
    I think I'll have to take it slow for now though, I have a Maths teacher who is trying to beat me to death with integration ([b(]) and I think my Chemistry is starting to suffer. If only I'd discovered this web-site at the beginning of the summer holidays, I'd have had loads of time to learn. :frown:
  11. Oct 21, 2003 #10
    It is the same Young. He was a bit of a genius and also deciphered the Egyptian Hierogliohics!! (I wish I could spell) This was something he was driven to do from an early age and so he learnt several ancient languages to assist him. On the discovery of the Rosetta stone (with the same script repeated in three languages) he managed to decipher what was written, and then master the Egyptian scripts.

    A pretty clever guy then....
  12. Oct 21, 2003 #11
    I wish I was that good at languages, that way I might have got a better mark than C in my GCSE's, it was a pass, but only just.

    Also, I might not have as much trouble with Maths as I do.
  13. Nov 27, 2003 #12

    Please give me the
    Definition, Why it occurs, examples of pccurence and the types of waves, of.....

  14. Mar 3, 2011 #13
    All optical effect with matter can be explained by quantum mechanics. diffraction and reflection mean the photons interact with atoms system with " elastic scattering. Well, this can be reguarded as a photon interact with a bounded state of electrons then the electrons go through a number of virtual transition and then go back to original state then emit the photon elastically. This is perturbation picture. Or any quantum process even dyson equation etc. It seems the dielectric constant is first order effect, but it include the higher order effect with EM wave as well, just like renormalization charge, mass...... This question is not simple indeed, many secondary school teacher don't understand the physical original and then use phenomenological way to explain.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook