From the five positions of object versus image in the previous diagrams, it should be clear what a convex lens could be used for. Some examples are:
• magnifying glass (Fig. 15.8) • spectacles • optical instruments requiring light-gathering functions such as telescopes and binoculars (Fig. 15.9).
STS
Spectacles first appeared around 1000 AD in Italy. They were used only by the very wealthy because lenses were so expensive to manufacture. After the invention of the printing press in the mid-fifteenth century, it became easier to produce books. Reading became more common and the availability of glasses increased.
FIG. 15.9 Telescopes use convex lenses
FIG. 15.8 Printers and designers use magnifying glasses with convex lenses to check print quality
Concave lenses
As in Chapter 13 Spherical mirrors, we need only show the formation of one type of image from a concave lens. This requires some basic rules in how light behaves in such a lens.
Rules of refraction in a concave lens
1. Any ray that comes into the lens parallel to the principal axis will emerge as if it is coming from the focal point on the same side (Fig. 15.10).
