REFLECTION, ANOLS or. The law of reflection is generally expressed by the asser tion, that " the angle of incidence is alwaye equal to the angle of reflection," and is thus explained: If in the accompanying figure A a plane surface, and a ball at D be impelled towards C, perpendicular to that surface, in the direction D C, it will rebound from the point C back towards D, in the same line C D; but if the ball be projected to the same point C, from any point E, in a direction not perpendicular to A B, it will rebound on the other side of D C, towards F, in such a man ner that E C D, which is termed " the angle of incidence," shall always be equal to F C D, " the angle of reflection." If instead of a ball, we suppose a ray of light to emanate from E, and fall upon C, it will be reflected in the same manner to the eye A F, along the line C F; in which direction only could an object, re flected from the point C, be visible. A pulse of air, which is sometimes a " ray of sound," follows the same law, and, if proceeding from E, would be reflected from C, directly in the line C F, in the points of which it would be heard, (if sufficiently strong,) as a reflected sound or echo. Some authors call E C B the
angle of incidence, and A C F that of reflec tion ; but the misnomer is of little cense quence, for they, too, are equal. When the reflection is made from a concave, or from a convex surface, the angles of incidence and reflection are still equal ; but they are mea sured by the tangent, or rather tangential plane, which touches the curve at the point on which the incident ray falls. We may add, that all terrestrial rays are divergent, as pro ceeding from a point ; but those of the sun are, on account of his immense distance, considered ae paralleL Convergent are such as meet in a focus, which they can only do by reflection or by refraction.