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Figure 9.16 The position, size and the nature of the image formed by
Figure 9.16
Figure 9.16
Figure 9.16
Figure 9.16
a convex lens for various positions of the object

Figure 9.17 Nature, position and relative size of the image formed by a concave lens
Figure 9.17
Figure 9.17
Figure 9.17
Figure 9.17
9.3.6 Sign Convention for Spherical Lenses
For lenses, we follow sign convention, similar to the one used for spherical
mirrors. We apply the rules for signs of distances, except that all
measurements are taken from the optical centre of the lens. According
to the convention, the focal length of a convex lens is positive and that of
a concave lens is negative. You must take care to apply appropriate
signs for the values of u, v, f, object height h and image height h′.

9.3.7 Lens Formula and Magnification

As we have a formula for spherical mirrors, we also have formula for
spherical lenses. This formula gives the relationship between object-
distance (u), image-distance (v) and the focal length (f ). The lens formula
is expressed as

1 1 1
− = (9.8)
v u f

The lens formula given above is general and is valid in all situations
for any spherical lens. Take proper care of the signs of different quantities,
while putting numerical values for solving problems relating to lenses.

Light – Reflection and Refraction 155

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