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Homework Help: Thins Lens Problem

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data

    (a) How far from a 50.0-mm-focal-length lens must an object be placed it its image is to be magnified 2.00X and be real?

    (b) What if the image is to be virtual and magnified 2.00X?

    2. Relevant equations

    Thins Lens Equation: 1/f = 1/do + 1/di

    Magnification Equation: M = hi/ho = -di/do

    3. The attempt at a solution

    I see that we're only given three pieces of information:
    • f=50.0mm
    • M=2.00
    • The image is real

    However, I don't know how to incorporate the magnification equation with the Thins Lens Equation.
  2. jcsd
  3. Feb 14, 2012 #2


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    Draw a ray trace - the answer is then obvious, and comes VERY quickley.
  4. Feb 15, 2012 #3


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    Have you drawn the ray trace yet?
  5. Feb 15, 2012 #4

    I have answered my own question mathematically.

    Since the magnification is 2.00, di = (2)(do). I just plugged it in the lens equation to find do.
  6. Feb 15, 2012 #5


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    Did you get both the real image and virtual image answers?
  7. Feb 15, 2012 #6
    Yes, a virtual image would have a di = -2(do) because the virtual image is upright and in front of the lens.
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