Thick Converging Lens
Diagram of a positive (converging) lens. The lensmaker's formula relates the radii of curvature, the index of refraction of the lens, the thickness of the lens, and the focal length.
Source
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"Lensmaker's equation."
http://en.wikipedia.org/wiki/Lensmaker's_equation%23Lensmaker.27s_equation
Wikipedia
GNU FDL.