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Math Help - image of the moon

  1. #1
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    image of the moon

    You wish to form an image of the moon, diameter 3500 km, a distance of 380 000 km away, with a reflecting telescope. If the image needs to be 4.5 cm in diameter, find the required focal length of the mirror in the telescope.
    Formulas are
    \frac{1}{f}=\frac{1}{di}+\frac{1}{do}
    \frac{hi}{ho}=\frac{-di}{do}
    F stands for focal point, di is distance to the image, do is distance to the object, hi is height of the image and ho is height of the object.

    \frac{4.5*10^-5}{3500}=\frac{-di}{380000}
    f=-4.88*10^-3
    \frac{1}{f}=\frac{1}{380000}+\frac{1}{-4.88*10^-3}
    f= -4.88*10^-3 km
    is this correct???
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  2. #2
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    Quote Originally Posted by iamanoobatmath View Post
    Formulas are
    \frac{1}{f}=\frac{1}{di}+\frac{1}{do}
    \frac{hi}{ho}=\frac{-di}{do}
    F stands for focal point, di is distance to the image, do is distance to the object, hi is height of the image and ho is height of the object.

    \frac{4.5*10^-5}{3500}=\frac{-di}{380000}
    f=-4.88*10^-3
    \frac{1}{f}=\frac{1}{380000}+\frac{1}{-4.88*10^-3}
    f= -4.88*10^-3 km
    is this correct??? Yes
    Since d_O is very large the image must be placed very near the focus. Therefore d_i = f.
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