It does not require a great amount of imagination to recognise that the double convex lens, that is the lens with two outwardly curved faces is little more than a pair of prisms placed base to base, or more accurately, a number of prisms so arranged as shown in the diagram. Parallel rays of light falling upon such an arrangement of prisms would be bent from their course, as shown by the arrows, and this is just what happens with a double convex lens. Now rays of light from an object, passing through a lens of this shape may follow any one of three courses, according to the position of the object with regard to the lens. In one position and one only the rays after passing through the lens will be parallel to one another, as shown in the diagram.
The only position of the object for the above to take place is when it coincides with a point known as the principal focus of the lens, conversely the parallel rays of light from the sun, after passing through a double convex lens, will come to a point at its principal focus.
Suppose now that the object be placed at a point beyond the principal focus of the lens, the light rays therefrom will, after passing through the lens, converge to a point thus:—
In the diagram O is the object and P the principal focus of the lens.
The third case occurs where the object is nearer to the lens than its principal focus, then the rays after passing through the lens, diverge and never meet.
We have already stated that when white light passes through a prism it is broken up into different coloured rays varying from violet to red. The reason for this is that all the light rays composing white light are not bent equally as they pass from one medium to another. The violet rays are bent the most, the indigo next, blue next, down to red, which is least bent. Once more, considering the double convex lens as made up of a number of prisms, let us represent, by a diagram, the course of parallel rays of white light through it.
A A′ represent the parallel rays of white light falling on the lens, L L′. The blue rays are bent more than the red, so the principal focus of the former is at C and of the latter at D. The consequence of this difference in bending of the various coloured light rays would be most serious in microscopic work were not means devised to overcome it. Objects for instance at C in our diagram would appear blue, at D they would appear red, whilst at E E′ though no single colour would predominate they would be illumined with many coloured rays, though less strongly than at C or D.