CHAPTER XXII. THE REFRACTION OF LIGHT.

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This term appears to be often confounded with that of reflection, and signifies the bending or breaking back of a ray of light (re, back, and frango, to break); and it will be remembered that when light falls on the surface of a solid (either liquid or gaseous) body, it may be reflected (re, back, and flecto, to bend), refracted, polarized, or absorbed. In the previous chapter the property of the reflection of light has been fully investigated, and in this one refraction only will be considered. It is a property which has been, and will continue to be, of the greatest practical utility in its application to the construction of all magnifying glasses, whether belonging to the telescope, microscope, magic lantern, or the dissolving views; or the minor refracting instruments—such as spectacles, opera-glasses, &c.; and it should be remembered that their magnifying power depends solely on the property of refraction.

If substances such as glass had not been endowed with this property, it would be difficult to understand how the great discoveries in the science of astronomy could have been made, or what information we could have gained respecting those interesting truths so constantly revealed by the aid of the microscope. Numerous instances might be quoted of the value of this latter instrument in the detection of adulteration, and the examination of organic structures. When so many talented and industrious scientific men are at work with this instrument, it is perhaps invidious to point to one singly, though we must make an exception in favour of Professor Ehrenberg, of Berlin, whose microscope did such good service in procuring undeniable proof of the Simonides' fraud; he has made use of it again to detect the thief that stole a barrel of specie, which had been purloined on one of the railways. One of a number of barrels, that should have contained coin, was found on arrival at its destination to have been emptied of its precious contents, and re-filled with sand. On Professor Ehrenberg being consulted, he sent for samples of sand from all the stations along the different lines of railway that the specie had passed, and by means of his microscope identified the station from which the sand must have been taken. The station once discovered, it was not difficult to hit upon the culprit in the small number of employÉs on duty there.

The simplest case of refraction occurs in tracing the course of a ray of light through the air, and into the medium water; in this case it passes from a rare to a dense medium, and the fact itself is well illustrated by the next diagram, in which the shaded portion represents water, and the paper that it is drawn upon the air. The line A B is a perpendicular ray of light, which passes straight from the air into and through the water, without being changed in its direction. The line C D is another ray, inclined from the perpendicular, and entering the water at an angle, does not pass in the straight line indicated by the dotted line, but is refracted or bent towards the perpendicular at d e.

Fig. 286. Fig. 286.

This fact reduced to the brevity of scientific laws is thus expressed:—When a ray of light falls perpendicularly on a refracting surface, it does not experience any refraction or change of direction. When light passes out of a rare into a dense medium, as from air into water, the angle of incidence is greater than the angle of refraction. And when light passes from a dense into a rare medium, as out of water into air, the reverse takes place, and the angle of incidence is smaller than the angle of refraction.

In order to illustrate these laws, a zinc-worker or tinman may construct a little tank, with glass windows in the front and sides, the latter being as deep as the half-circle described on the back metal plate of the tank, which of course rises higher, in order to show the full circle; this should be japanned white, and a perpendicular and horizontal black line described upon it—the whole, with the exception of the circle, being japanned black. If the Duboscq lantern is arranged with the little mirror, as described in fig. 276, page 287, the ray of light may be thrown perpendicularly, or at an angle, through the water, and the actual breaking back of the ray of light is rendered distinctly apparent. (Fig. 287.)

Fig. 287. Fig. 287.

a. Duboscq lantern. b. The mirror. b c. The incident ray. c d. The refracted ray. e f. Tank, containing water up to the horizontal line of the circle.

The refraction of light is also well displayed by Duboscq's apparatus, with the plano-convex lens, and a brass arrow as an object, with another double convex lens to focus it. When a good sharp outline of the arrow is obtained on the disc, a portion of the rays of light producing it may then be truly broken out or refracted by laying across the brass arrow a square bar of plate glass. (Fig. 288).

Fig. 288. Fig. 288.

a. Rays of light from the electric light. b. The cap, with figure of arrow cut out. c. The bar of plate glass. d. The double convex glass to focus e, the image on the disc, and portion refracted at b.

There are many simple ways in which the refraction of light is displayed, such as the apparent breaking of an oar where it enters the water, or the remarkable manner in which the bottom is lifted up when we look, at any angle, through the clear water of a deep river or lake; the latter circumstance has unhappily led to most serious accidents, in consequence of children being induced by the apparent shallowness of the water to get in and bathe. Fish, again, unless seen perpendicularly from a boat, always appear nearer than their true position, and the Indians, when they spear fish, always take care to strike as near the perpendicular as possible; experienced shots know they must aim a little lower and nearer than the apparent position of a fish in order to hit it.

Having learnt that light is bent from its course, it might be supposed that all objects looked at through plate glass should appear distorted; but it must be remembered that the sides of the glass being nearly parallel, an equal amount of refraction occurs in every direction—so that, unless the window is glazed with uneven wavy glass, the object, for all practical purposes, does not apparently change its position, being neither moved to the right or the left, or upward or downward. In order to bend the rays of light in the required direction, the glass must be cut into certain figures called prisms, plane glasses, spheres, and lenses, some of which are shown in the annexed cut. (Fig. 289.)

Fig. 289. Fig. 289.

It would be tedious to trace out, by a regular series of diagrams, the passage of light through the variety of combinations of lenses; and as the plane, convex, and concave surfaces have been examined with respect to their effect on the reflection of light, they may be referred to again with regard to their influence in refracting light. In the latter it will be found that convex and concave lenses have just the opposite properties of mirrors; thus, a convex lens receiving parallel rays will cause them to converge to a focus. (Fig. 290.) The case of short-sighted persons arises from too great a convexity of the eye, which makes a very near focus; and that of old people is a flattening of the eye, by which the focus is thrown to a greater distance. The remedy for the latter is a convex spectacle-glass, whilst a concave lens is required for the former, to scatter the rays and prevent their coming to a point too soon.

Fig. 290. Fig. 290.

a b. A double convex lens. c is a ray of light, which falls perpendicularly on a b, and therefore passes on straight to f, the focus. d b. Rays falling at an angle on a b, refracted to focus, f.

The action of a concave refracting surface is again the opposite to a concave reflecting surface—the former disperses the rays of light, whilst, the latter collects them. A concave lens, as might be expected, produces exactly the contrary effect on light to that of a concave mirror. (Fig. 291.)

Fig. 291. Fig. 291.

a b. A double concave lens. c., is a ray of light which falls perpendicularly on a b, and passes through without any alteration of its course. d d. Rays falling at an angle on a b, are refracted and diverged.

These facts are well shown with the aid of the lantern and electric light. The rays of light are refracted in a visible manner when received on a concave or convex lens, provided a little smoke from paper is employed, as in the mirror experiments. (Fig. 292.)

a. The electric light. b. The lens.

Bearing these elementary truths in mind, it will not be difficult to follow out a complete set of illustrations explanatory of the construction and use of various popular optical contrivances.


                                                                                                                                                                                                                                                                                                           

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