Page:Popular Science Monthly Volume 5.djvu/45

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35
THE CROOKED COURSES OF LIGHT.

in form and size to the real objects in front of them. Fig. 2 represents the formation of an image of a candle in a common looking-glass. The reflection is shown as limited to the pencil of rays emitted by the highest point of the flame. The reflected rays which enter the eye are seen to be divergent like the incident rays, so that if they were produced backward they would meet at a point forming the image at the top of the flame. As all surfaces are made up of points, and each point of the object is reflected in the same manner, it is clear that the image formed by a plane mirror must symmetrically represent the object.

Fig 4. Fig 5.
PSM V05 D045 Snell law of refraction.jpg
Appearance of a Stick in Water. Snell's Law of Refraction.

But light-rays may be turned from their direct course in another way. When a beam passes obliquely from one transparent medium to another of different density, as from air to water or glass, its direction is changed and it is said to be refracted. This is illustrated in a very simple manner by Fig. 3, in which a ray of the sun, entering through an aperture in a dark room and received on the surface of water in a glass vessel, is seen to be broken as it were at the surface and bent downward.

A familiar experiment illustrating the same principle is to put a coin upon the bottom of an empty, opaque vessel, while the spectator places himself so that it is just hidden by the vessel's edge. If water be now poured into it, the bottom of the dish will appear to rise, and the coin will come in sight. The pencil of rays thus undergoes a sudden bend at the surface of the water, and reaches the eye by a crooked course, the effect of which is, that the spectator sees round or behind the obstacle. Fig. 4 shows how an inclined stick, partially immersed in water, presents a broken appearance. Transparent substances differ in this refracting power. Liquids exhibit it in a much higher degree than gases, and, as a general rule, the denser of two substances manifests the greater refracting effect. Hence it is common to speak of the change in the ray as it passes from a denser into a rarer medium, or the reverse.