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Questions with mirrors

  1. Jun 4, 2005 #1
    A light ray in air enters and passes through a square block of glass. What can be stated with regard to its speed after it emerges from the block?
    a) Its speed is less than when in glass
    b) Its speed is less than before it entered glass
    c) Its speed is same as that in glass
    d) Its speed is same as that before it entered glass


    In order to see a huge right-side-up image of yourself, you should use a concave mirror and place your face:
    a)inside the focal point
    b)at the focal point
    c)between the focal point and twice the focal poing
    d)at twice the focal point
    e)beyond twice the focal point

    I think the answer to this one is a, but I am not positive.

    Please help!
  2. jcsd
  3. Jun 4, 2005 #2
    I'd have to say the answer to the first one is 'b' since the speed of light is constant(for the material that it is in) and if both sides of the block are the same material, like air, then the speed will be the same on either side.

    For the second one, the answer is a. It will be an enlarged, virtual, image. Here's an article to help explain what goes on with images in a concave mirror. http://www.physicsclassroom.com/Class/refln/U13L3e.html
  4. Jun 5, 2005 #3
    Light, in a vacuum, travels at just under 3 x 108ms-1. As soon as a medium is put in the way, its speed decreases. It may not be by a lot but you can see the difference when it enters a glass block, as it appears to 'bend' the light. The speed of light in the same medium is the same and the answer is not 'b'.

    So, if the medium is the same on the exit side of the block as on the enterance side of the block (e.g. it travels in air before and after it is in the glass block), what do you think we can say??? Remember, light in the same medium is the same speed.

    The Bob (2004 ©)
    Last edited: Jun 5, 2005
  5. Jun 5, 2005 #4


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    So I don't think you mean "b".
  6. Jun 5, 2005 #5
    So would its speed be the same as that in glass or would its speed be the same as that before it entered glass? I'm still confused.
  7. Jun 5, 2005 #6
    Let me quote myself:

    To the question:
    So you have narrowed it down to the same speed as in the glass block or the speed before it enters the glass block. Well when the light comes out of the block it enters a medium. What is this medium?? When was it last in this medium??

    The Bob (2004 ©)
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