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All gases, including air, refract the rays of light according to their density. The light, on entering the atmosphere, is refracted according to its density; and as that portion of the atmosphere nearest the surface possesses the greatest density, it must also possess the greatest refractive power. From this cause the sun and other celestial bodies are never seen in their true position unless they happen to be vertical, and the nearer they are to the horizon, the greater will be the influence of refraction in altering their apparent places. Morning does not occur at the instant of the sun's appearance above the horizon, nor does night commence the instant the sun disappears below it. Both at morning and evening the rays proceeding from the sun below the horizon are refracted by passing through the atmosphere, or bent down towards the surface of the earth. As the density of the air diminishes from the surface of the earth, there is not that sudden change of direction we observe in a stick partly immersed in water; but a ray of light proceeding from any celestial body describes a curve, being more and more refracted at each step of its progress through the atmosphere. This also applies to light received from distant objects on the surface of the earth which are higher or lower than the eye.

Total Reflection.-When light passes from a medium to one more refractive it will always be refracted; but not so when it passes into a less refractive medium, as when it passes from water or glass into air. In this case the angle of incidence is limited, beyond which refraction cannot take place. B M C represent a hollow globe half full of water. A ray of light coming from L to A, being normal to the surface of the globe, experiences no refraction on entering the globe; but on reaching A, if the angle of

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incidence L A C is small enough, it will be refracted from the normal B A, and pass out into the air in some such direction as A R. But if the angle be increased, as / A C, then, the refraction decreasing, we shall have the ray passing out at right angles to B A, along A M. The angle / AC is then the limiting angle of refraction. From water to air this angle is 48° 35'. From glass to air, 41° 48'. It is evident then if the angle exceeds 48° 35', as is the case with the ray n A, that it will not pass out of the water at A,—in other words, will not be refracted at all, but it will be totally or internally reflected in some such direction as A r.

This kind of reflection at the surface which separates two media is called internal or total reflection. It is called total reflection because all the light is reflected, which is not the case under any other circumstances of reflection, no matter how carefully the reflecting surfaces may be polished.

Total reflection may be observed by the following simple experiment.

Let A represent a coin placed beyond the limiting angle of refraction. Rays passing to the surface of the water in the glass vessel are internally reflected,

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Fig. 45.

as if the surface of the water was a mirror with the face downwards. They strike the eye at B, and the image of the coin is seen in the direction B C.

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Mirage. The term "looming" is applied by sailors to a curious optical deception by which objects come into view, though materially altered as to their shape and position. The French call it "mirage," the Italians, "fata morgana." It often happens that ships appear as if painted in the sky, and not resting upon the water. Rocks and sands appear raised above the surface. The Swedes long searched for an illusory island of this sort, which they saw from a distance placed between the isles of Aland and the coast of Upland. In the unusually hot summer of 1868 a "mirage" of the coast of France, extending some miles, was seen from the opposite English shores.

These phenomena are due to the varying density of the different strata of the atmosphere lying near the surface of the earth. Rays, therefore, proceeding from distant objects and passing through these strata of different density, will be unequally refracted, and proceed in a curvilinear direction until they strike an upper stratum at such an angle as shall produce

internal reflection; and in this way an object behind a hill or below the horizon may become visible, and appear suspended in the air.

Suppose the rays of light from a ship (Fig. 46) below the horizon to pass through layers of the atmosphere of different density, as A B C.

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Fig. 46.

On attaining the extreme of layer C the angle of refraction passes the limit, and the rays become totally reflected. Then, as an object always seems to be in the direction in which the last rays proceeding from it enter the eye, an inverted image will be seen in the direction m n. Again, the rays may be so reflected as to give an upright image of the object. For example, the rays may be reflected so as not to cross each other, and then the position of the image will be upright. Occasionally at sea, but more commonly on the hot sandy plains of Egypt and Africa, ordinary objects are seen by means of horizontal rays, and below them inverted images of the objects, just as reflections of trees, &c., on the banks of a stream are seen by an observer on the opposite shore. These are caused by a reversal of the ordinary arrangement of the strata of the atmosphere. Consequently, rays in proceeding from the object are refracted in such a

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manner as to reach the limiting angle, and then become totally reflected, as in the preceding example.

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The diagram (Fig. 47) will assist the student in understanding the latter phase of the "mirage."

These effects may be illustrated by heating an iron rod and then placing it in a horizontal position. The air in contact with the upper surface of the iron will be more rarefied than that at some distance above it, and thus the order of density will be to a small height inverted; consequently, in looking horizontally along the bar to any object a little height above it, its direct image will be seen by means of horizontal rays, and an inverted image will be seen below it by reflected rays.

A ray of light passing through a medium whose two sides are parallel will suffer little change by refraction, since the second face or surface exactly compensates for the refractive effect of the first.* Let A B (Fig. 48) represent a section of a plate of glass,

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Although some little difference of position takes place, owing to the thickness of the glass, yet glass is generally so exceedingly thin that this forms no appreciable change in the position of the object.

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