Having learned so much about light, we come to the most important fact of all, as far as the action of light concerns microscopic work. When rays of light travel, from a substance like air into a substance like water, they are bent out of their straight course. Without any desire to introduce a number of unfamiliar words, we may venture to remark that, any substance through which light passes is called a medium. Some media are clearly more dense, more compact or solid—dense is the proper word—than others. Water is more dense than air and glass than either. The bending of light rays is known as refraction. So now we may state our second law a little more concisely, thus:—When light passes from a medium into one more dense, or vice versa, it is refracted, and the more dense the medium into which or from which the light passes the greater the refraction.
A diagram and an experiment should make matters clear. Suppose AB is a ray of light traveling in air and that it falls on a sheet of water, WXYZ, the ray will be bent along BC and its course from air to water may be represented by ABC. Suppose again, WXYZ represents, not water but glass; as glass is more dense than water the course of the ray AB is represented by ABD, it is refracted or bent to a greater extent than the ray which passed from air into water.
For our experiment we need only plunge a stick into water and notice that, owing to this property of light, the stick appears bent, from the point where it comes into contact with the surface of the water.
Some of us may be old enough to remember that once, on either corner of nearly every mantlepiece, there stood an ornament of doubtful utility from which there hung a dozen or more glass prisms. Now the only beauty about these otherwise hideous contraptions was to be seen when light played upon them. Then patches of violet, green, yellow and red were thrown upon neighbouring objects. White light, ordinary sunlight that is to say, is really composed of various colours—violet, indigo, blue, green, yellow, orange and red—which, when combined together, make light as we know it. When white light passes through a prism of glass, it is not only bent out of its course, but broken up into all these colours. A prism, as we all know, when examined at either end, is seen to be triangular in shape. Putting aside for a moment the question of the breaking up of light into its component parts, the path of a ray of light through a prism is shown in the diagram. As the ray passes from air into glass it is bent, because glass is more dense than air; it is bent once more on leaving the prism because air is less dense than glass.
Now lenses are made of various shapes, and those with two outwardly curved surfaces are known as double convex lenses. A double convex lens is usually made with both its surfaces equally curved and in the finer optical work great care is taken to ensure that this is the case. For certain purposes, however, as we shall learn in a moment, one or other of the faces only may be much more curved than its companion and this may be carried to such an extreme that one face is flat, the lens is then known as plano-convex. Lenses may also have inwardly curved faces, if both are of this design they are called double concave; if one face is flat and the other inwardly curved they are known as plano-concave. There are other combinations, for example, one face may be inwardly curved and the other outwardly curved, but the four kinds we have described are all that need trouble us.
