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Angle of Deviation - 2 Mirrors

  1. May 14, 2010 #1
    1. The problem statement, all variables and given/known data
    Hi, my teacher asked me to conduct an experiment on the question "How does the angle of deviation depend upon the angle between two mirrors?" - We weren't given much else on the topic. I know how to find the angle between two mirrors, but finding the angle of deviation of the light ray is what I'm lost at.

    2. Relevant equations

    Law of Reflection?

    3. The attempt at a solution
    I came up with my own solutions to try and find a method to calculate angle of deviation. Anyway, here is the diagram which I created to hopefully shed some light on the situation

    [PLAIN]http://img696.imageshack.us/img696/6097/phys1k.jpg [Broken]

    As you can see, θi is the incoming light ray (just a name for it...) and θf is the final, or outgoing ray. Now, as seen, θf (the outgoing ray) has been reflected straight back in the same direction as the original ray. Do I just say the angle of deviation is 360, or does it have something to do with the angles made at the mirrors (m1 and M2). θ1+θ2 = 180, so is that the angle of deviation?


    Now in the image below (which I again made, myself), if you only adjust one of the mirrors (I changed M2), you get a different story. Now, as seen the outgoing light ray continues on in the same direction as the incoming light ray. Is the Angle of Deviation Zero degrees here? Or do I add up the angles between M1 and M2? Which gives me 180? But how can the Angle of Deviation be 180 when the light ray simply keeps going on in the same direction?

    [PLAIN]http://img404.imageshack.us/img404/7955/phys2.jpg [Broken]

    I'm lost as to how to find the Angle of Deviation of a light ray after reflection of 2 mirrors...
    Please help. Thanks !!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 14, 2010 #2
    A ray of light that is incident on to the surface of a plane-mirror is reflected with the angle of incidence equal to the angle of reflection. Suppose that the ray had continued, through the mirror, in a straight line it would make an angle θ with the surface of the mirror. The total angle between the straight-line path and the reflected ray is twice the angle of incidence. This is called the deviation of the light and measures the angle at which the light has strayed from its initial straight-line path.

  4. May 14, 2010 #3
    If that's the case then, how do I apply that to 2 mirrors? I understand that concept for 1 mirror... But how to apply it to 2, I'm lost.
  5. May 14, 2010 #4


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    Homework Helper

    Angle of deviation is the angle of reflected ray with respect to the incident ray. It is nothing to do with the number of the mirrors and their arrangement in between.

    Now you can find out the result.
  6. May 14, 2010 #5
    So with my first image posted up, if I compare the reflected ray to the incoming ray, Is the angle of deviation 360*? and for the second image is just simply zero*?
  7. May 14, 2010 #6


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    Homework Helper

    In the first case the angle of deviation is 180*. It can't be more than 180*. The ray of light is not a rotating vector.
  8. May 15, 2010 #7
    Thanks mr. Rajrendra Lakshmanapathy Bhat... your help has been great. So, from this, I take it you just compare the two angles together (incident & final-reflected rays) and see the angle made between them... So for my second image, the Angle of Deviation is 0*, right? Again, thanks for the help mate
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