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Homework Help: Reflection of light

  1. Oct 24, 2009 #1

    Three objects P, Q, and R, are viewed through a plane mirror AB. When an obstacle is moved towards the mirror, which object "disappears" (i) first; (ii) last?

    Diagram. I will describe it in words. The mirror is horizontally placed. Q is placed 10 cm directly above edge A of the mirror. R is placed 20 cm directly above edge A. P is 10 cm (horizontally) to the left of A.

    2. Relevant equations


    3. The attempt at a solution

    I'm thinking of the positions of the points of incidence of the incident rays coming from the objects. However, when I draw ray diagrams, it doesn't seem so. Maybe the angle of incidence also matters - the smaller, the faster the object "disappears"? I am completely stumped.
  2. jcsd
  3. Oct 24, 2009 #2

    Doc Al

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    Staff: Mentor

    Your description is not complete. You didn't define A and B or left and right. (A diagram would really help here.)

    Here's a trick for dealing with plane mirrors. Each real object has an image exactly on the other side of the mirror. Then you can think of the mirror as a window for viewing those images. If your view of that window is blocked, so is the image.