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Help with mirrors?

  1. Aug 4, 2006 #1
    A ray of light strikes a plane mirror at a 45° angle of incidence. The mirror is then rotated by = 21° 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°?


    For this one I kind of just split up the angles making it more into a geometry problem. Since the entire system turned 21 degrees..the given angle would change by about 24 degrees..but that isnt correct..


    When a diverging lens is held 10 cm above a line of print, as in Figure 26.29, the image is 4.1 cm beneath the lens. What is the focal length of the lens?


    This question is a little confusing to me. I think its saying the hobject = 10cm and the himage = 10-4.1..which can then be used to deduce the focal length equals 3.71..but thats also wrong.

    Any help? Thanks

    Last edited: Aug 4, 2006
  2. jcsd
  3. Aug 4, 2006 #2


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    Staff: Mentor

    For -1-, keep in mind that there's probably a factor of 2 involved. Think about the case where a laser strikes a mirror at normal incidence, so that the beam is reflected back to the laser. Now rotate the mirror by 45 degrees -- how much is the reflected ray rotated?
  4. Aug 4, 2006 #3
    THanks for your help..I finally managed to figure out the first one..still having some trouble with the second..the thing is the question is misleading..I'm not even sure what exactly I'm given in the problem..

    any help guys?
  5. Aug 4, 2006 #4
    i disagree. i think the problem is saying that the distance between the object and the lense is 10cm and the distance btwn the image and the lense is 4.1.

    So, distance of object = p = 10cm (if we define the left side be the front) and distance of image = i = -4.1cm (its negative since its a virtual image).

    use the thin lense equation and solve for f, the focal length

    f^-1 = p^-1 + i^-1
    f = (p^-1 + i^-1)^-1
    = (10^-1 + -4.1^-1)^-1
    = -6.95 cm

    this makes since that f is negative since it is a diverging lense
    Last edited: Aug 4, 2006
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