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Why does light bend,not just slow down?

  1. Jun 3, 2012 #1
    When a light beam passes from an optically denser medium to an optically less dense medium(and vice versa),the direction and the speed of light changes.
    My question is why light must bend when it goes from one medium to another.
    Say,it goes from air to water. Water is optically denser than air, which means the speed of light in air is higher than the speed of light in water.Why doesn't the light just slow down in water, why does it have to bend in the process?
  2. jcsd
  3. Jun 4, 2012 #2


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    If you search google images for "refraction" you will find diagrams explaining this.


  4. Jun 4, 2012 #3
  5. Jun 4, 2012 #4
    Is this phenomenon related to force/torque/direction of velocity?As force is required to change the direction of object.Seems that there isn't any "force" in this situation.
    I just can't imagine why the difference in velocity will cause light rays to bend.
    (I have not learnt waves yet.I have learnt heat,mechanics and light.)
  6. Jun 4, 2012 #5
    The law of refraction can be derived from the Fermat's Principle/Principle of least time, which states that light follows the path of least time.
  7. Jun 4, 2012 #6
    Wave moves into the direction that the wave fronts of the wave are facing.

    And the wave fronts are mangled, when different parts of wave fronts have different velocities.

    Here is a picture of three wave fronts: | | |
    That wave moves either into this direction: --> , or into this direction: <--

    Here is a better wave front picture:
    Last edited: Jun 4, 2012
  8. Jun 4, 2012 #7
    One picture can say more than a thousand words. :smile:
    Perhaps useful to add that it applies the Huygens construction:
    When the OP understands how that works, then he can answer his own question. :smile:

    Jadelamlam, you only need to understand that a wave propagates at constant speed (depending on the medium) in all directions, as you can see when you throw a stone in the water; and that a wave front (green straight lines in the drawing here above) can be thought of as the sum of many little waves (grey half circles).
    Next, you have to take a pencil (not a pen, for surely you'll use your eraser!) and try for yourself.
    Last edited: Jun 4, 2012
  9. Jun 4, 2012 #8


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  10. Jun 4, 2012 #9
    Yes, that's a good example if presented as an axle with two independently rolling wheels connected to it.
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