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Spherical Lenses 29

Fig. 3.5: The principal foci of thin spherical lenses

Since, the medium on both sides of the lens is the same, i.e., air, parallel
rays incident on the lens from the opposite direction, i.e., from the right in
Figure 3.4 will be refracted in an identical way. There is, therefore, a
principal focus on each side of the lens, equidistant from the nodal point.
The first principal focus F , is the point of origin of rays which, after
1
refraction by the lens, are parallel to the principal axis. The distance F N is
2
the first focal length. Incident rays parallel to the principal axis is converged
to or diverged from the second principal focus – F . The distance F N is the
2
2
second focal length. By the sign convention, F has a positive sign for convex
2
lens and negative sign for the concave lens. Lenses are designed by their
second focal length. Thus, the convex or converging lenses are also called
“Plus Lenses” and are marked with “+”, while concave or diverging lenses
are also called “Minus Lenses” and are marked with “–”. If the medium on
either side of the lens is the same, i.e. air, then F = F . However, if the
1
2
second medium differs from the first, e.g. in case of contact lenses, then F 1
will not be equal to F .
2
DIOPTERIC POWER OF LENSES, VERGENCE

Lenses of shorter focal length are more powerful than lenses of longer focal
length. Therefore, the unit of lens power, the diopter, is based on the
reciprocal of the second focal length expressed in metres, gives the vergence
power of the lens in diopters (D).