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Refraction and glass blocks- help please

  1. Sep 1, 2006 #1
    Two non-parallel light rays initially converge to a single point on a screen. A rectangular block of glass is now placed somewhere in front of the screen, in the path of the light rays, so that the glass surface is parallel to the screen. Where is the new convergence point of the rays?

    1, Behind the screen.
    2,On the screen (unchanged).
    3, Inside the glass block.
    4, Toward the glass block, in front of the screen.


    I put inside the glass block, because i drew out the picture and i dont see the two rays converging at all because it is a glass block and not a concave lens. I drew a picture with the two rays entering then refracting towards the normal, and then refraction again when going out of the block, but this time away from the normal since its going from the glass to the air, and the speed of light in air is faster so the rays move away from the normal, which means that they dont converge on the screen at all. but that also means that they only converge at the initial point of contact which is when they first hit the block, but there is no option saying on the block, it just says inside the block. please help....
     
  2. jcsd
  3. Sep 2, 2006 #2

    andrevdh

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    The light rays exiting the block will run parallel to the rays entering the block. The exit rays is offset to one side with respect to the entry rays. This happens because the refraction angle at the first interface (air -> glass) becomes the incident angle at the second interface (glass -> air) and we therefore get the same refraction angle in air (from Snell's law) at the second interface as when the beam entered the glass block. Usually one find such a diagram in a basic physics handbook.
     
    Last edited: Sep 2, 2006
  4. Sep 2, 2006 #3

    HallsofIvy

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    You don't need a concave lens, you were told that the rays were already converging to a point on the screen. You are, in fact, right that the block does not act as any kind of lens and, so, does not change the convergence of the rays.
     
  5. Sep 2, 2006 #4
    And so the rays stay on the screen (unchanged) ?
     
  6. Sep 2, 2006 #5

    andrevdh

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    The rays are displaced off to one side, but they are still parallel to the original direction. Try drawing it.
     
  7. Sep 2, 2006 #6
    Thanks for your help :-)
     
  8. Sep 3, 2006 #7

    andrevdh

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    It's a pleasure.
     
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