Focal length
In the diagrams, F is the principal focus and f the focal
length.
The focal length
and principal focus of a lens are defined as follows:
The point where parallel light is brought to a focus is called the PRINCIPAL FOCUS of the lens and the distance of this point from the lens is called the FOCAL LENGTH of the lens.
The focal length is a way of describing the strength of
the lens. The shorter the focal length the more sharply curved the sides of the lens will
be, and in the case of a convex lens it will converge light more rapidly.
Power of a lens
The
power of a lens (what opticians use to describe the strength of a lens when you go to have
your eyes tested) is defined as:
Power of a lens = 1/[focal length in
metres]
Converging lenses have positive powers, diverging lenses have negative
powers.
A lens with a power of + 5 D (dioptres) is a convex lens with a focal length
of 20 cm. One with a power of -10 D is a concave lens with a focal length of 10
cm.
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