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Fig. 39 a - Principal focus of a converging lens. The incident rays are

parallel to the principal axis POF; b - Principal focus of a diverging lens

When the bounding surfaces are only convex, the lens has a long focal length. In order that all the rays may come to a point after leaving the lens, the beam of light must be restricted to a narrow bundle near the principal axis of the lens. For an extended beam the outer rays will not pass through the same point as the rays near the axis.

When the surfaces of the lens are concave instead of convex, the lens makes the rays that pass through it more divergent, and for this reason it is known as a diverging lens. In Figure 39b parallel rays are incident on a concave lens. On entering the lens, the rays are bent toward the normal as before, and on leaving they are bent away from the normal. In this case, however, the emerging rays are bent away from the prin­cipal axis. They appear on leaving the lens to come from a point F behind the lens. When the incident rays are parallel to each other and to the principal axis, this point from which the rays appear to come on leaving the lens is the principal focus. This is only an apparent focus because the light does not really come from it, but the effect on the left-hand side of the lens is the same as if the light actually came from this point behind the lens. This kind of focus from which the light appears to come is a virtual focus. It is to be carefully distinguished from a real focus through which the light actually goes.

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