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The Epic Interactive Encyclopedia 1997
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1992-09-03
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The distance from the centre of a spherical
mirror or lens to the focal point. For a
concave mirror or convex lens, it is the
distance at which parallel rays of light are
brought to a focus to form a real image (for
a mirror, this is half the radius of
curvature). For a convex mirror or concave
lens, it is the distance from the centre to
the point at which a virtual image (an image
produced by diverging rays of light) is
formed. With lenses, the greater the power
(measured in dioptres) of the lens the
shorter its focal length.