340. =Reflection of Light.=--Light, like sound and heat, is reflected in straight lines when it strikes upon any resisting substance. We can see this to be the case when it strikes upon any smooth and plane surface. And it is true of light, as it is of heat, that the angles of incidence and reflection are equal. Thus if _c_, Fig. 227, be a reflecting surface, and _b c_ a line perpendicular to it, then a ray of light, _d c_, will be reflected in the line _c a_, and the angle of incidence, _d c b_, will be equal to the angle of reflection, _b c a_. 341. =How we See.=--We see the various objects around us by the light which is reflected from them. Every point of every surface that we see reflects rays or vibrations of light to our eyes. Thus if we see a person there are rays of light reflected into our eyes from every part of him. These rays form an image of him in the back part of each eye, and it is by this image that we see him, as will be explained in full in another part of this chapter. Reflected light is painting the images of objects in the eye every moment in great abundance and variety. If a speaker have an audience of a thousand persons all looking at him, his image is at the same time in two thousand eyes, and in each of these two thousand images every motion and every changing expression are faithfully depicted. [Illustration: Fig. 228.] 342. =Mirrors.=--That reflected light does thus form images of objects you see in the common mirror. The image formed in it of any object comes from the light reflected from that object into the glass. Then in seeing the image light is reflected from it into the eye, there to form a similar image, though of much less size. By using two or more mirrors the reflections of the image can be multiplied, and by some arrangements of them to a very great extent. That the image appears to be at the same distance beyond the surface that the object is before it, is owing to the fact that the reflected rays come from the glass at the same angle that the incident rays strike upon it. This may be shown from Fig. 228 (p. 263). Suppose _m m'_ is a looking-glass, and an arrow, A B, is before it. Rays of light come from it at all points to the glass. We will take only two of these rays at each end of the arrow. The ray A _g_ will be reflected to the eye at the same angle in the ray _g o_, and the ray A _f_ will be reflected in the ray _f_ E. And the reflected rays will have the same rate of divergence as the incident rays. The same can be shown in regard to rays from B or any other point on the arrow. Now if the lines _o g_ and E _f_ be extended, they will meet at the point _a_, which is at the same distance behind the mirror as A is before it. The same thing can be shown of the rays from B or any other point. Therefore the image of the arrow will appear to the eye to have the same relative position behind the glass that the arrow itself has before it. [Illustration: Fig. 229.]