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Physics mirror problem

  1. Apr 27, 2009 #1
    1. Light from a distant star is collected by a concave mirror. How far from the mirror is the image of the star if the radius of curvature is 150 cm?



    2. 1/f = 1/do + 1/di



    3. 1/f = 1/do + 1/di
    f = 150/2 = 75 cm
    di = dof/do - f
    = do(75)/do- 75

    I don't know what do is or if there is one. If there is one, can you tell me what it is. Thank You!!
     
  2. jcsd
  3. Apr 27, 2009 #2

    mgb_phys

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    Re: Mirrors

    The object distance is at infinity (or pretty close!) so the light arrives parallel
     
  4. Apr 27, 2009 #3
    Re: Mirrors

    um.. what do u mean by the light is parallel? oh and i know that the answer is 75 cm but i don't know how to get it.
     
  5. Apr 27, 2009 #4

    mgb_phys

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    Re: Mirrors

    One of the rules of a concave mirror is that the focal length is half the radius of curvature.

    Rules of optics
    Parallel light in goes out through the focus
    Light in through the focus goes out parallel

    draw a sketch with the focal point, the centre of curvature an the light ray
     
  6. Apr 27, 2009 #5
    Re: Mirrors

    oh ok thank you!
     
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