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Homework Help: Thermal Physics

  1. Apr 6, 2014 #1
    Lens & Disk

    The image of the sun is projected on the disk using a lens of radius r<<d and focal length f<<d (d is the distance between the sun and the disk.. The magnification of the lens is M`f/d, so that the image completely fills the disk R`=MR. When the disk reaches thermal equilibrium, what is its temperature?

    I don't really know where to start, so a hint would help a lot.

    Thanks
     
  2. jcsd
  3. Apr 6, 2014 #2

    Simon Bridge

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    How would you do the problem if there was no lens?
     
  4. Apr 7, 2014 #3
    I've already done that part.

    Computed the flux in and compared it with the flux out of the disk. got a temperature of $T_{disk}=T\sqrt{\frac{R}{d}}$ where d is the distance from the sun to the disk and R is the radius of the sun.
     
  5. Apr 7, 2014 #4

    Simon Bridge

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    ... good.

    So what does the lens do in terms of the solar flux that gets to the disk?
     
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