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 this same perpendicular is called the angle of reflection. Observation and experiment have taught us that light is always reflected in such a way that the angle of reflection equals the angle of incidence. Light is not the only illustration we have of the law of reflection. Every child who bounces a ball makes use of this law, but he uses it unconsciously. If an elastic ball is thrown perpendicularly against the floor, it returns to the sender; if it is thrown against the floor at an angle (Fig. 61), it rebounds in the opposite direction, but always in such a way that the angle of reflection equals the angle of incidence. [Illustration: FIG. 60.--The ray _AC_ is reflected as _CD_.] [Illustration: FIG. 61.--A bouncing ball illustrates the law of reflection.]