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Homework Help: Light/mirror questions.

  1. Nov 21, 2012 #1
    1. The problem statement, all variables and given/known data
    1. A student wishes to place an object in front of a concave mirror in order to produce an image one-half the object's size. If the focal length of the mirror is 5.0cm, how far from the mirror should the object be placed? ANS : 15 cm.

    2.An object is placed 2.0cm beyond the centre of curvatur of a concave mirror. If the magnification of this object is 0.30 what is the radius of curvature of this mirror. ANS: 1.7cm

    2. Relevant equations

    1/f = 1/do + 1/di
    m = hi/ho = -di/do

    3. The attempt at a solution

    1. I believe these are questions where you have to combine two formulas, i dont know how to go about it, im not asking for full work, if oyu decide to show full steps, im trying to figure it out and i cant seem to do it, can someone start me off on the right track? Or if im overthinking it put me on the right track.

    1) 1/2ho/ho= f-do/do
    which would be -5, which isnt right.
  2. jcsd
  3. Nov 21, 2012 #2


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    Homework Helper

    What do you mean with equation (1)? ho/ho=1 and do/do=1, so eq(1) is equivalent to 1/2=f-1 which is obviously wrong.

  4. Nov 22, 2012 #3
    well hi is half of ho which makes it 1/2ho, so i replaced hi with 1/2ho
    and i replaced di with f-do from

  5. Nov 22, 2012 #4


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    Gold Member

    No, 1/f = 1/do+1/di. That's quite different.
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