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Please Explain Me Refraction of light

  1. Aug 13, 2007 #1
    Please Explain Me Refraction of light !!!!

    Helo everyone,
    I have gone through my textbooks which only comes to the conclusion that since the light travels at different velocity in different medium so it bends at the interfaces. This is something i can't realy understand. It doesnot make anything clear to me. I have searched this forum for help regarding this. what i get is this

    ->" Speed of photons is same i.e C and it is a apprent decrease in velocity"
    Whatever it is how does that leads to bending?
    ->Then to explain snell's law i got another fermat's law which again does not seem to help me. It says light has to travel the shortest path but why should it ?

    I guess there are other theories or better explainations at the atomic level.
    Pls help me.

    And again even if there's some theory to learn in order to understand this why should the textbooks for undergraduate level assume that everyone is just going to mug up this bending phenomenon.(Since it doenot give more explainations)


    Sorry for my english....
  2. jcsd
  3. Aug 13, 2007 #2


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    I will try and explain in simple English
    The speed of light is 'c' only in vacuum.
    In a material the light is absorbed by an atom and then re-emitted a short time later, it then travels at 'c' the short distance to the next atom.
    How quickly the atoms in a material can absorb and re-emit the photon decides the speed of light in that material.
    In simple books this is described as if light had a slower average speed in a material.

    Fermat's law is a brilliantly simple description of how light behaves - and you can work out the whole of optics and a few other fields of physics from it. BUT laws of physics don't describe WHY something happens, only how.
  4. Aug 13, 2007 #3
    thanks mqp_phys for reply. but i can understand from your and other's replies(in other threads) why light has a apparent less velocity in different mediums. but i cant understand why it leads to bending of light..

    Can this why not be answered ?
  5. Aug 13, 2007 #4
    i can't even visualize the fact .. i can see a light ray or wave or photon getting a lesser velocity in water( previously in vacuum ) and then driven by some unknown agent, bends towards or away from the normal.
  6. Aug 13, 2007 #5
    fermats principle= light is trying to get from point a to point b in the least amount of time... you learned that the fastest route between two points is a straight line.... but that is simply not true if the light goes faster in one medium than in another... if the light starts out in a slow medium, it wants to get out of that medium as fast as possible, so it takes a path that gets it into the fast medium even if that path is not toward the final objective, then after it gets to the faster medium, it changes direction and heads for the target, point b... it is essentially trying to divide up its travel time so that it spends the majority of time in the fast medium, and as little time as it can in the slow medium...
  7. Aug 13, 2007 #6


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    The simple explanations of why light bends ( one edge of the wavefront slowing down first, columns of soldiers marching accross a field ) are all wrong.
    The correct way to think about it, is that light takes all the possible paths from a to B some go the shortest distance in air and then the longest distance in water, some take the longest distance in air and then the shortest distance in water and everythign in between. In our universe we only detect the ones that take the shortest path, which is the path exactly described by Snell's law and refraction.
    This quantum-electrodynamics explanation is more true than the simple ones but seem to make less sense in the real world!
    So nothing 'makes' the light bend, we just only detect the light which takes the path that looks as if it had bent!
  8. Aug 13, 2007 #7
    True, and the reason is that the phases for the all the possible paths that go to point B add with a net positive intensity if point B is chosen according to Snell's Law. If point B is chosen somewhere else, the phases add destructively with a net intensity of zero.
  9. Aug 14, 2007 #8

    Is it ? Is it all about interference and diffraction. then how does the less apparent speed of light in the medium affect the constructive/destructive superposition. I have also read somewhere that due to scattering of light the direction of light changes.Please make this a bit clearer.( if its true in the first place)
  10. Aug 14, 2007 #9

    Claude Bile

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    Sort of from a photonic point of view. From a classical (wave) perspective - which is more appropriate - elastic (Rayleigh or Mie but not Raman) scattering is simply another form of diffraction.

    Refraction is due to the requirement that E must be continuous across a boundary. To see this visually, draw a straight interface on a piece of paper. On one side of the interface draw a set of wavefronts. Now on the second half of the interface, draw a second set of wavefronts that a) have a shorter wavelength than the first set (hence the wavefronts will be closer together) and b) join up with the original wavefronts, so wherever the wavefronts from the first set intersect with the interface, the second set must also intersect (this is a graphical representation of the requirement that E must be continuous across a boundary). You will notice firsthand that in order to satisfy these requirements, the direction of the wave (which can be found by taking the normal to the wavefronts) must change. Trigonometric analysis of this scenario reveals Snell's law.

    Don't think of photons changing direction, that is the wrong picture. Photons in this scenario are entirely "wave-like".

  11. Aug 15, 2007 #10
    hi,mgb_phys,i have one more question.you said 'In a material the light is absorbed by an atom and then re-emitted a short time later',this idea seems plausible.because you can see the light still go directly through the medium,if your re-emitting(i think it just like to be scattered)mechanism is right,i think the photon should go through all the directions it can reach and cannot go through one direction from the original.

  12. Aug 15, 2007 #11


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  13. Nov 12, 2008 #12
    Re: Please Explain Me Refraction of light !!!!

    Claude Bile's explanation above is correct, but the phenomenon is best understood with a graphic diagram: http://alienryderflex.com/refraction

    Why light exists as discrete particles (photons) is another subject, separate from why refraction occurs. Refraction applies to all waves, such as water-surface waves, or sound waves.
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