Thin Lens
Thin lenses have the same focal length on either side. (a) Parallel light rays entering a converging lens from the right cross at its focal point on the left. (b) Parallel light rays entering a diverging lens from the right seem to come from the focal point on the right.
Source
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"OpenStax College, Image Formation by Lenses. December 29, 2012."
http://cnx.org/content/m42470/latest/#import-auto-id1625442
OpenStax CNX
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