REFLECTION, that which is reflected, or produced by being reflected; an image given back from a reflecting surface. Also the act or habit of turning the mind to something which has already occupied it; thoughtful, attentive, or continued consideration or deliberation; meditation, thought.
A surface on which a beam of light falls may be either rough or smooth. If it be rough, the greater part of the incident light is irregularly scattered by the innumerable surface facets, so as to be reflected or dispersed in all direc tions; if it be smooth, a proportion (but never the whole) of the incident light is regularly reflected or turned back in definite paths. A smooth, dustless mir ror is not visible to an eye outside the track of rays reflected from it. If the polished surface be that of a transparent substance (e. g., glass) optically denser than the medium conveying the light to it, comparatively little light is reflected; but the more oblique the incidence, the smoother the polish, and the greater the difference between the optical density of the glass and that of the medium in which it is immersed, the greater the proportion reflected. Thus less light is reflected from glass under water than from glass in air; and conversely, if the light travel in the denser medium and strike the bounding surface between it and a rarer medium—as where light ascending through water strikes its upper free surface—it will, if its ob liquity of incidence exceed a certain limit, be almost totally reflected; the small loss that ensues arising wholly from absorption, while no light is trans mitted into the air above. This may be shown by holding a clear tumbler of water above the head; the image of ob jects beneath is seen reflected in a bright mirror surface; and a phenome non of the same order is seen on thrust ing a test tube containing air below the surface of water, when it will appear to have a luster like quicksilver. If the reflecting surface be that of an opaque body the bulk of the incident light is reflected, a percentage being lost by absorption. What has been said about light applies ether undulations of all kinds, and therefore the theory of reflection has general reference to ra diant heat, light, actinic radiation, and electro-magnetic undulations. Reflection arises in all cases from a difference in the transmissibility of ether disturb ances on the two sides of the bounding surface.
On reflection from polished surfaces we have, so far as regards the directions of the reflected rays, the following laws observed: (1) The incident "ray," the normal (i. e., a line drawn perpendicu
lar) to the surface at the point of inci dence, and the reflected "ray" all lie in one plane, the "plane of incidence"; and (2) the angle of incidence (the angle which the incident "ray" makes with the normal to the reflecting surface) is equal to the angle of reflection (the corre sponding angle between the normal and the reflected "ray"). These laws apply equally to ether waves of all lengths, and therefore to light of all colors; and they also hold good whatever be the shape of the surface. If the surface be plane their application is simple; and if the surface be curved we have, in effect, to consider the curved surface as made up of indefinitely small facets, to each of which the above laws can be applied. The geometrical consequences of these laws make up what used to be called catoptrics, that part of geometrical optics which deals with reflection; and this coincides in its propositions with that part of kinematics, which gives an account of the reflection of waves. Here the other waves (using the term "waves" in its most general sense) are assumed to travel through optically ho mogeneous media, and can consequently be traced out by imaginary lines drawn at right angles to the wave fronts or along the directions pursued by the waves, these imaginary lines being called "rays." Plane Reflecting Surfaces.—(1) Rays which are paralled to one another before striking a plane reflecting surface are parallel after reflection. (2) If light diverging from or converging toward a point be reflected from a plane mirror, it will appear after reflection to diverge from or converge toward another point situated on the opposite side of the mirror and at an equal distance from it. If, on the other hand, the course of the light is such that the rays appear before reflec tion to converge on the second point, they will after reflection actually pass through the first one. (3) A consequence of the preceding proposition is that when an ob ject is placed before a plane mirror the virtual image is of the same form and magnitude as the object, and at an equal distance from the mirror on the other side of it. The right hand of the image taken as looking toward the mirror, is necessarily opposite to the left hand of the object; so that no one ever sees him self in a single plane mirror as others see him or as a photograph shows him, but he sees all his features reversed.