Convex Lens
Rays of light entering a converging lens parallel to its axis converge at its focal point F. (Ray 2 lies on the axis of the lens. ) The distance from the center of the lens to the focal point is the lens's focal length f. An expanded view of the path taken by ray 1 shows the perpendiculars and the angles of incidence and refraction at both surfaces.
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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