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Homework Help: Astrophysics Question!

  1. Oct 15, 2013 #1
    1. The problem statement, all variables and given/known data

    You are lost in a forest. You have become separated from your friend. At your eye level, only tree
    trunks block your view of distant parts of the forest (there are no leaves and branches at eye level to
    block your view, only tree trunks). The trees are spread randomly throughout the forest, but with a
    typical spacing of about 3 m, and the tree trunks have typical diameters of 20 cm. You can hear your
    friend calling, but you can't see her. Approximately how close do you need to get to each other before
    you will see your friend?

    2. Relevant equations

    Mean free path=1/n*sigma=1/n*pi*r^2
    Optical Depth T=x/1/n*sigma=n*x*sigma

    3. The attempt at a solution

    Not really sure where to go with this to be honest.... I dont see how I have enough information/what assumptions to make

    Obviously we can solve for Sigma since we have the diamter, but then we dont know how big the forest area/volume is so we cant find n (make an assumption?)

    Thanks for any help
  2. jcsd
  3. Oct 15, 2013 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    You know the mean spacing of the trees... you don't need to know the overall size of the forest if you do it in terms of densities. Perhaps think this way: what is the average size the forest needs to be to block your view of the outside?
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