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Homework Help: Q's about Refraction

  1. Jul 5, 2004 #1
    Why do shorter wave lengths of light (ie:blue light) refract more than longer wave lengths (ie:red light)?

    and why does light bend not just slow down? what is causing the light to follow and bend with the other light? what is this attraction force caused by?
    :confused: :confused: :confused:
    Last edited: Jul 5, 2004
  2. jcsd
  3. Jul 5, 2004 #2
    Sounds like you're talking about dispersion; like in a prism. The index of refraction differs slightly for different wavelengths (colors). (The index of refraction is how quickly light travels thru the medium as compared to air). So if blue light has a wavelength of 400 nm and an index of refraction of 1.4698, and if red light has a wavelength of 700 nm and an index of refr. of 1.4503, and we apply Snell's Law, we find the angle at which the red light is dispersed is greater than that with which the blue light is dispersed.

    The reason the light bends and doesn't just slow down is that it is abruptly forced to travel thru a different medium with a different index of refraction.
  4. Jul 6, 2004 #3
    i understand Snell's law, and all the math and everything behind refraction and dispersion, and the fact that light DOES refract (different colours differently etc) but i dont actually know WHY any of that happens. WHY the light refracts, WHY the different wavelengths refract differently etc etc. i dont think we were ever actually taught how it happens, just the fact that it does and details about it. i dont remember.
  5. Jul 6, 2004 #4
    i wasn't talking about dispersion :P
    What i ment was what Brennen is saying.
    but also why it bends!
    For it to bend it has to have an attraction force between photons. Refraction was explained to me by a moving shopping trolly. Pushed from a concreat path onto some grass, it BENDS towards the normal because of the different times each wheel encounters the grass, slowing it down, but also because of the "attraction force"(the metal bar joining th wheels) between the right wheel and the left wheel.
    But if you push two bikes from concrete onto grass they will just slow down, not change angle and not "bend".
  6. Jul 6, 2004 #5


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    The bending is required for continuity in the electric field. It is very hard to prove at a high-school level.

    Light is essentially an oscillating electromagnetic wave. The electric field must be continuous, and conserved when the light strikes a surface. The fields of the reflected and refracted beams must be consistant with the incident beam. When you work through the problem with these restrictions, you come up with Snell's law.

    There is also a neat trick called the "path of least time". Pick any two points with a prism between them, and the path that light will take to get from one to the other is defined by Snell's law. No big deal, right? But it is also the path of least time. Remember, light travels slower in the prism. The bent pathway reduces the time spent in the prism.

  7. Feb 20, 2010 #6
    It actually is not too difficult to prove why waves refract when propagating through different mediums, and some of the proofs are in accordance with a high school level. There are calculus proofs of snells law as well as proofs using simple geometry, and honestly neither of them are that difficult. Waves refract, or "bend", because of the fact that their wavelengths change as they go from one medium to another. First think of a wave as a plane wave, as if they are composed simply of parallel wave fronts. Then, one can easily see that when the wave length changes, in other words the distance between the parallel wave fronts, in order to remain parallel the wave fronts must change direction. Thus, when a wave goes from a rarer medium to a denser one, and the wave length shortens, the wave refracts towards the normal to the surface, and when a wave goes from a denser to a rarer medium it bends away from the normal. Try drawing it out on paper, it will immediately become obvious.
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