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Homework Help: Optics: radius of a central disk of light

  1. Aug 9, 2010 #1
    1. The problem statement, all variables and given/known data

    A telescope objective is 12cm in diameter and has a focal length of 150cm.

    Light of mean wavelength 550nm from a distant star enters the scope as a nearly collimated beam.

    compute the radius of the central disk of light forming the image of the star on the focal plane of the lens

    2. Relevant equations

    [tex] \theta_{min\frac{1}{2}} = \frac{1.22*\lambda}{D} [/tex]

    Radius of disk =[tex] L * \theta_{min\frac{1}{2}} [/tex]

    3. The attempt at a solution

    after showing the proof for those equations all I did was solve for the radius of the disk by plugging in the numbers,

    [tex] \frac{1.22 * (550*(10^{-9}))}{\frac{12}{100}m}*\frac{150}{100}m [/tex]

    putting into a calculator I got 8.3875*10^-6, approximately 8.39*10^-6

    however the answers show 8.39*10^-4,

    what am I missing?
  2. jcsd
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