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Telescope focal length homework

  1. Jan 2, 2010 #1
    Hey guys, Im really stuck on my homework review and really require help! Someone please help ! :(
    1. The problem statement, all variables and given/known data
    Question 1: A telescope has a focal length of 18m. If the suns diameter is about 1.39 x 10^9m and its distance from Earth is 1.49 x 10^11, how large is the telescopes image of the sun.
    2. A converging mirror produces an image twice the size of the obbect. If the focal length is 20cm, find the object distance.

    2. Relevant equations
    1/f = 1/do + 1/di
    m = -di/10 = hi/10
    - FOR BOTH

    3. The attempt at a solution
    f = 18m
    do = 1.49 x 10^11
    ho = 1.39 x 10^9

    1/f = 1/do + 1/d1
    1/f - 1/do = 1/di
    -0.56 = di
    m = - (di)/do
    = - (-0.56m / 1.49 x 10^11)
    3.76 x 10 = m
    m = hi / ho
    3.76 x 10 ^10 = hi / 1.39x 10 ^...

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 2, 2010 #2


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    Homework Helper

    Q. 1
    Image of the sun is framed at the focus of the objective. If you take two diametrically opposite rays from the sun to from the image at the focus, the optical center of the objective, ho and hi form two similar triangles. So
    do/ho = di/hi. di is the focal length of the objective. Find hi.
    Q.2 Show your attempt.
  4. Jan 3, 2010 #3
    Hey there.. we havent quite learned anything about the formula you told me about where di = f. the hi/di. Isn't there any other way i can possibly do it?
  5. Jan 3, 2010 #4


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    Gold Member

    rl.bhat just used simple geometry to derive that equation, so I'm fairly certain that it's probably the solution your professor/teacher is looking for.
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