SECTION II.. _the Refraction of Light._

The Refraction of Light.

A ray of light, in its passage through any transparent medium, travels in a straight line as long as the density of the medium continues unchanged. But if the density varies, becoming either greater or less, then the ray is refracted, or bent out of the course which it originally pursued. The degree to which the refraction or bending takes place depends upon the nature of the new medium, and in particular upon its density as compared with that of the medium which the ray had previously traversed. Hence Water refracts light more powerfully than Air, and Glass more so than Water.

The following diagram illustrates the refraction of a ray of light.

The dotted line is drawn perpendicularly to the surface, and it is seen that the ray of light on entering is bent towards this line. On emerging, on the other hand, it is bent to an equal extent away from the perpendicular, so that it proceeds in a course parallel to, but not coincident with, its original direction. If we suppose the new medium, in place of being more dense than the old, to be less dense, then the conditions are exactly reversed,--the ray is bent away from the perpendicular on entering, and towards it on leaving.

It must be observed that the laws of refraction apply only to rays of light which fall upon the medium at an angle: if they enter perpendicularly--in the direction of the dotted lines in the last figure--they pass straight through without suffering refraction.

Notice also, that it is at the surfaces of bodies that the deflecting power acts. The ray is bent on entering, and bent again on leaving; but whilst within the medium it continues in a straight line. Hence it is evident that by variously modifying the surfaces of refractive media the rays of light may be diverted almost at pleasure. This will be rendered clear by a few simple diagrams.

In the figures given below, and in the following page, the dotted lines represent perpendiculars to the surface at the point where the ray falls, and it is seen that the usual law of bending towards the perpendicular on entering, and away from it on leaving the dense medium, is in each case correctly observed.

Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3, consisting of two prisms placed base to base, causes rays before parallel to meet in a point; and conversely, fig. 4, having prisms placed edge to edge, diverts them further asunder.

The various forms of Lenses.--The phenomena of the refraction of light are seen in the case of curved surfaces in the same manner as with those which are plane.

Glasses ground of a curvilinear form are termed Lenses. The following are examples.

Fig. 1 is a biconvex lens; fig. 2, a biconcave lens; and fig. 3, a meniscus lens.

As far as regards their refractive powers, such figures may be represented, nearly, by others bound by straight lines, and thus it becomes evident that a biconvex lens tends to condense rays of light to a point, and a biconcave to scatter them. A meniscus combines both actions, but the rays are eventually bent together, the convex curve of a meniscus lens being always greater than the concave.

The Foci of Lenses.--It has been shown that convex lenses tend to condense rays of light and bring them together to a point. This point is termed "the focus" of the Lens.

The following laws as regards the focus may be laid down:--

That rays of light which are pursuing a parallel course at the time they enter the Lens are brought to a focus at a point nearer to the Lens than diverging rays. The rays proceeding from very distant objects are parallel; those from objects near at hand diverge. The sun's rays are always parallel, and the divergence of the others becomes greater as the distance from the Lens is less.

The focus of a Lens for parallel rays is termed the "principal focus," and is not subject to variation; this is the point referred to when the focal length of a Lens is spoken of. When the rays are not parallel, but diverge from a point, that point is associated with the focus, and the two are termed "conjugate foci."

In the above diagram A is the principal focus, and B and C are conjugate foci. Any object placed at B has its focus at G, and conversely when placed at C it is in focus at B.

Therefore, although the principal focus of a Lens (as determined by the degree of its convexity) is always the same, yet the focus for objects near at hand varies, being longer as they are brought closer to the Lens.

Formation of a Luminous Image by a Lens.--As the rays of light proceeding from a point are brought to a focus by means of a Lens, so are they when they proceed from an object, and in that case an image of the object is the result.

The above figure illustrates this. The size of the image varies with the distance of the arrow from the glass--being larger and formed at a point further from the Lens as the object is brought nearer. The refracting power of the Lens also influences the result--lenses of short focal length, i. e. more convex, giving a smaller image.

In order that the course pursued by pencils of rays proceeding from an object may be easily traced, the lines from the barb of the arrow in the last figure are dotted. Observe that the object is necessarily inverted, and also that those rays which traverse the central point of the Lens, or the centre of the axis, as it is termed, are not bent away, but pursue a course either coincident with, or parallel to, the original, as in the case of refracting media with parallel surfaces.