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Homework Help: Focusing with the Human Eye

  1. Sep 15, 2007 #1
    1. The problem statement, all variables and given/known data
    Joe is hiking through the woods when he decides to stop and take in the view. He is particularly interested in three objects: a squirrel sitting on a rock next to him, a tree a few meters away, and a distant mountain. As Joe is taking in the view, he thinks back to what he learned in his physics class about how the human eye works.

    Joe first focuses his attention (and his eyes) on the tree. The focal length of the cornea-lens system in his eye must be __________ the distance between the front and back of his eye.
    A - Greater Than
    B - Less Than
    C - Equal To

    2. Relevant equations

    3. The attempt at a solution
    I don't know where to begin. I believe focal length on one side of a lens is the same as the focal length on the other side. But this problem talks about this cornea-lens system, a "compound lens system," which throws me off.

    But here's my flow of logic, please feel free to comment.
    -Our eyes are converging lenses
    -Our eyes can change the focal length in order for us to focus on objects at different distances.
    -The object is a real inverted image after it passes through our eye
    -The image must focus on our fovea in order for it not to be a "sharp image"
    -The fovea is at the back of the eye.

    So if the fovea is at the back of our eye, which is where the image needs to be focused, that means the focal length is B) equal to that distance?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 15, 2007 #2


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    Gold Member

    Your eyes can change squeeze their lenses around their perimeters, changing their curvature and thus their focal length. Does this help?
  4. Sep 15, 2007 #3
    Not so much turbo,

    I don't know much about the eye and after studying a handful of diagrams, I still don't know.

    Can we change the curvature of our eye so the focal lengths ("outside" and "inside" the eye) are not equal? Is it possible? All my readings and notes show converging lenses where there is only one focal length, never two different ones. Is it safe to assume that's how it for the lenses in an eye? The focal length can be adjusted but its always equal to?

    The Badger
  5. Feb 2, 2010 #4
    The answer is less then, the focal length could be any distance as long as it's not greater than the space within the eye.

    Sorry I cannot provide a proper explanation
  6. Feb 2, 2010 #5
    The "focal length" is the distance at which PARALLEL RAYS FROM INFINITY would converge. When looking at the tree, are you focusing parallel rays from infinity?
  7. Feb 2, 2010 #6


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    Homework Helper

    The distance between the front and back(fovea) of the eye remains constant. Focusing is done by adjusting the curvature of the lens which is nearer to the fovea that the front of the eye.
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