The ray A B falling obliquely on the horizontal mirror, is reflected upwards at the same angle in the direction B C. This may be proved geometrically by placing a graduated circle in a vertical position in the plane A B C, when we shall find that the angle A B D formed by A B (the incident ray) with the perpendicular D B is equal to the angle formed by this perpendicular line and the reflecting ray B C. You may also prove in the same way that these three lines are all in the same vertical plane.
Fig. 13.—Reflection from Plane Surfaces.
Let us now examine the effects of light reflected from plane surfaces. We must first, however, notice a certain optical illusion to which we are continually falling a prey, almost without our knowledge. We always fancy objects to be in reality in the place where we see them, and, in spite of our having already enumerated a large number of these deceptions, we must still add one more to the list. In reality we rarely see objects in the place where they really are; for if by the effect of reflection, refraction, or any other cause, the rays of light are made to deviate from their course, we no longer see the object from which they proceed in its real position, but in the direction taken by the luminous pencil at the moment of entering the eye.
Fig. 14.—Refraction.
Fig. 15.—Experimental Proof of Refraction.
For instance, if the ray A B is bent during its passage to the eye at B, and consequently reaches it in the direction B C, it is at A´, and not at A, that we shall see the object from which it proceeds. Every ray of light which passes out of a medium of a certain density into another of a different density is bent from its primary course, or, in scientific language, it is refracted. The experiments we made in a former chapter on the properties of the prism are founded on this principle. The law may be easily illustrated by allowing a ray of light to fall upon the surface of a vessel of water, as shown in the preceding figure.
