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Homework Help: Mirrors (concave) + questions on lens and mirrors

  1. May 3, 2009 #1
    1. The image of the moon is formed by a concave mirror whose radius of curvature is 4.20 m at a time when the moon's distance is 3.8 x 10^5 km. What is the diameter of the image of the moon if the diameter of the moon is 3480 km?

    2. 1/f = 1/do + 1/di
    M = hi/ho = - di/do

    3. 1/f = 1/do + 1/di
    di = dof/do - f
    = 3.8 x 1065(0.0042)/3.8 x 10^5 - 0.0042
    = 0.0042 km

    hi/ho = - di/do
    hi= ho (- di/do)
    = 3480(-0.0042/3.8 x 10^5)
    = -3.8 x 10^-5 km

    I'm not sure if this is right. I also have some questions how do you know if it is concave or convex (lens/mirrors if the book doesn't say it), what's the difference between mirrors and lens, and how do you tell if its a converging lens or a diverging lens whats the difference between them. Thank You!! :]
  2. jcsd
  3. May 3, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    4.2m is the radius of curvature, it is not the focal length. See your textbook for how f depends on R.

    A lens works by refraction of transmitted rays, while a mirror works by reflection.

    We know this is a concave mirror because the problem statement says so.

    More generally:

    A positive focal length indicates a concave mirror, and convex or converging lens.

    A negative focal length indicates a convex mirror, and concave or diverging lens.
  4. May 3, 2009 #3
    oh ok i see.
    i have two more questions:
    What happens if the problem says that it is a convex mirror and gives a positive focus do we change that to negative or if they give a negative focus do we change that to a positive?

    What happens if your trying to solve for the focus on a concave mirror and get a positve focus do I change that to a negative or leave it as it is?

    I'm kind of confused on this and I want to be able to understand it for my test tomorrow.
    Thank You!!
  5. May 3, 2009 #4


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    Staff Emeritus
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    The focal length is negative for a convex mirror, so:
    • if they give a positive focus, change that to negative
    • if they give a negative focus, keep it as negative

    A concave mirror has a positive focal length, so if you get a positive focus keep it positive.
    A convex mirror has a negative focus, so if you calculate a positive focus then something is wrong. Check your work.

    You're welcome. Good luck!
  6. May 3, 2009 #5
    Thank You again!! :]
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