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posted by  bittu236 on 7/23/2008 8:54:52 AM  |  status: Live  

OPTICS....LIFESAVER GUARANTEED

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 An image of the moon is focused onto a screen using a converging lens of focal length f= 35.8 cm. The diameter of the moon is 3.48×106 m, and its mean distance from the earth is 3.85×108 m. What is the diameter of the moon's image? 

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posted by Mystic Monkey on 7/24/2008 2:09:59 PM  |  status: Live
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the ratio of the focal length to the distance to the moon is the same as the ratio of the diameter of the image to the diameter of the moon, or:
 
    diam of image / diam of moon   =   focal length / dist to moon
 
  diam of image  =  diam of moon * focal length / dist to moon  =
 
                          =  3.48 x 106  *  0.358  / 3.85 x 108  =
 
                             =  0.003236 meters  =  3.24 mm   is the diam of the image
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