103.  Good and Bad Mirrors.  As we walk along the street, we frequently see ourselves reflected in the shop windows, in polished metal signboards, in the metal trimmings of wagons and automobiles; but in mirrors we get the best image of ourselves.  We resent the image given by a piece of tin, because the reflection is distorted and does not picture us as we really are; a rough surface does not give a fair representation; if we want a true image of ourselves, we must use a smooth surface like a mirror as a reflector.  If the water in a pond is absolutely still, we get a clear, true image of the trees, but if there are ripples on the surface, the reflection is blurred and distorted.  A metal roof reflects so much light that the eyes are dazzled by it, and a whitewashed fence injures the eyes because of the glare which comes from the reflected light.  Neither of these could be called mirrors, however, because although they reflect light, they reflect it so irregularly that not even a suggestion of an image can be obtained.

Most of us are sufficiently familiar with mirrors to know that the image is a duplicate of ourselves with regard to size, shape, color, and expression, but that it appears to be back of the mirror, while we are actually in front of the mirror.  The image appears not only behind the mirror, but it is also exactly as far back of the mirror as we are in front of it; if we approach the mirror, the image also draws nearer; if we withdraw, it likewise recedes.

104.  The Path of Light.  If a mirror or any other polished surface is held in the path of a sunbeam, some of the light is reflected, and by rotating the mirror the reflected sunbeam may be made to take any path.  School children amuse themselves by reflecting sunbeams from a mirror into their companions’ faces.  If the companion moves his head in order to avoid the reflected beam, his tormentor moves or inclines the mirror and flashes the beam back to his victim’s face.

If a mirror is held so that a ray of light strikes it in a perpendicular direction, the light is reflected backward along the path by which it came.  If, however, the light makes an angle with the mirror, its direction is changed, and it leaves the mirror along a new path.  By observation we learn that when a beam strikes the mirror and makes an angle of 30 deg. with the perpendicular, the beam is reflected in such a way that its new path also makes an angle of 30 deg. with the perpendicular.  If the sunbeam strikes the mirror at an angle of 32 deg. with the perpendicular, the path of the reflected ray also makes an angle of 32 deg. with the perpendicular.  The ray (AC, Fig. 60) which falls upon the mirror is called the incident ray, and the angle which the incident ray (AC) makes with the perpendicular (BC) to the mirror, at the point where the ray strikes the mirror, is called the angle of incidence.  The angle formed by the reflected ray (CD) and