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Homework Help: Finding distance of image from mirror given focal length

  1. Sep 9, 2016 #1
    1. The problem statement, all variables and given/known data
    An object is at a very large distance from a concave mirror of focal length 40cm. Find the distance from the image to the mirror. Is the image real or virtual?

    2. Relevant equations
    1/u + 1/v = 1/f

    3. The attempt at a solution

    1/u + 1/v = 1/40

    The image has to be real because it must be outside the focus if it is very far from the mirror, right?
    It seems to me like I haven't got enough information to solve the problem. How would I go about doing it? The answer from the key is 40cm.
  2. jcsd
  3. Sep 9, 2016 #2


    User Avatar
    Homework Helper

    What do you get if you put in a very large number (say, 10000cm) for the object distance? What happens as that distance gets even farther?
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