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Homework Help: Focal length of a lens

  1. Dec 14, 2007 #1
    I just have one question regarding the focal length of a lens. I had a test today and there was a fill in the blank section regarding how the focal length of a lens is determined (there was two empty spaces.) Wouldn't there be three possible answers. The curve of the lens, the index of refraction, and the thickness of the lens? On the test I only stated the curve of the lens and the thickness..
  2. jcsd
  3. Dec 14, 2007 #2
    Usually it's safe (and common) to use a thin-lens approximation. I'd say the best two answers are index of refraction and curve of the lens, but thickness of the lens is technically correct also.
  4. Dec 14, 2007 #3
    that's what I thought, I'll likely get that one wrong though due to my poor reasoning
  5. Dec 14, 2007 #4

    Shooting Star

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    In high school or thereabouts, only thin lenses are dealt with. The thickness does not matter much.

    Suppose I hold an imaginary lens made of air in front of my eyes. Would I see anything different? But for a lens made of glass, I'll see differently. This logic is enough to answer that the ref index of the lens matters.
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