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Homework Help: What is the general angle of theta?

  1. Jun 14, 2010 #1
    1. The problem statement, all variables and given/known data

    Two plane mirrors are joined along side at a right angle. Both are perpendicular to the table top, like walls meeting at a corner.

    For a general angle of theta, determine the direction of the reflected ray which leaves the double mirror?

    2. Relevant equations

    3. The attempt at a solution
    this is what i said:

    For a general angle of incidence theta the direction of the reflected ray which leaves the double mirror is the same as the angle that enters the double mirror. From the example given the light enters the double mirror on the left at a 45 degree angle. It then bounces off the mirror to the mirror at the right and reflects at a 45 degree angle from that mirror.

    This is wrong though

    Thank you
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 14, 2010 #2


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    Well, it is definitely true that light which enters one of these right angle mirrors will reflect back in the same direction it came from, as long as the direction of the incoming ray is within the 90° opening of the mirror (and assuming the problem is limited to the plane of the table). Maybe your answer was misinterpreted. Or maybe (though I find this unlikely) you were also supposed to account for rays coming from other directions, e.g. behind the mirror, or just barely outside the 90° opening (so that it reflects off one mirror but not the other).
  4. Jun 14, 2010 #3


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    It could be your answer is correct, but you were expected to show why it's true rather than just assert it.
  5. Jun 14, 2010 #4
    Make a drawing :)

    Attached Files:

  6. Jun 15, 2010 #5
    let's call the mirror that the incomming ray initially hits mirror 1, and the other can be called mirror 2. the angle theta between the incomming ray and mirror 1 will equal the angle between the outgoing ray and mirror 2.
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