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Homework Help: Light rays

  1. Feb 24, 2006 #1
    I need some help with light rays. Ive been sick and missed the last few days so I need a little help. I have some questions that I have to submit online by tomorrow. I was wondering if someone could help.

    A ray of light strikes a plane mirror at a 45° angle of incidence. The mirror is then rotated by = 10° into the position shown in red in the drawing, while the incident ray is kept fixed.
    (a) Through what angle does the reflected ray rotate?

    (b) What is the answer to part (a) if the angle of incidence is 60° instead of 45°?

    Is there some equation for this one? I wasnt sure if you just add 10°. I dont think thats it. I dont have my physics book either so I cant really read through what we have been doing.
    A candle is placed 15.0 cm in front of a convex mirror. When the convex mirror is replaced with a plane mirror, the image moves 8.0 cm farther away from the mirror. Find the focal length of the convex mirror.
    on this one i was trying this with the 1/Do + 1/Di = 1/f but wasnt getting anything out of it.

    I really need an answer tonight. I appreciate any help. Thanks.
    Last edited: Feb 24, 2006
  2. jcsd
  3. Feb 24, 2006 #2


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    HINT (a): Rotating the mirror also rotates the surface normal.
  4. Feb 25, 2006 #3
    One thing on part a of the question. Through what angle does the reflected ray rotate?
    I guess the phi symbol didnt show up because it was an image. If you look at the symbol on the picture, it shows that image to be between the red and blue lines. Does that mean that it would just be 10? or is it 45?
  5. Feb 25, 2006 #4
    iam doing an investigation on the inverse law using light. i want to know why can a flourescent tube act a line source? why is it that when you get too far the is acts a point source?
  6. Feb 25, 2006 #5
    I got #2, but I dont know about the first one. I have some guesses but I only have 1 submission. I only have 2 hours left before I have to turn it in so if someone could help me that would be great.
  7. Feb 25, 2006 #6


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    The original reflected ray is 45 degrees from the normal and if the plane rotates by 10 degrees then the original reflected wave is 35 degrees from the new normal. However, the new normal is 10 degrees off the old one so the incident ray is now 55 degrees from the new normal. Since the new reflected wave must also be 55 degrees from the new normal then its direction must be 55 - 35 = 20 degrees from the original reflected ray.
  8. Feb 25, 2006 #7
    Thanks a lot! I understand it now. :biggrin:
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