Reflection
that which is reflected, given back from a reflecting surface.
Also the act or habit of turning the mind to something which has already 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 directions; 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 mirror is not visible to an eye outside the polished surface be that of a transparent track of rays reflected from it. If the 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 obliquity of incidence exceed a limit, be almost totally reflected; the small loss that ensues arising wholly from absorption, while no light is transmitted into the air above. This may be mitted into the air above. This may be water above the head; the image of objects beneath is seen reflected in a non of the same order is seen on thrustbright mirror surface; and a phenomecertain ing a test tube containing air below the surface of water, when it will appear to 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 equally to ether undulations REFLECTION of all kinds, and therefore the theory of reflection has general reference to radiant heat, light, actinic radiation, and electro-magnetic undulations. Reflection arises in all cases from a difference in the transmissibility of ether disturbances 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 perpendicular) to the surface at the point of incidence, 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 corresponding 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 curved 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 homogeneous 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."
Pata 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 reflection 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 object 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 REFLECTION 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 himself in a single plane mirror as others see him or as a photograph shows him, but he sees all his features reversed. (4) When two mirrors are placed parallel to one another, light from an object between them is reflected back and fore, so as to appear on each occasion of reflection as if it came from images more and more remote from the mirrors.
On each occasion the course of the rays of light is the same as if the virtual image behind the mirror had been a real object; and a new virtual image is produced, apparently as far behind the reflecting mirror as the virtual object had been in front of it. If the mirrors were perfectly plane and parallel, and if they reflected all the light which fell on them, an observer between the mirrors would see in this experiment (which is called the endless gallery) an indefinite number of images. A variation of this experiment, carried out with mirrors not parallel to one another, but inclined at an angle which is some aliquot part of 180°, gives the principle of the kaleidoscope. (5) When a beam of light is reflected from a mirror and the mirror is turned through a given angle, the reflected beam is swept through an angle twice as great.
This principle is utilized in the construction of many scientific instruments, in which the reflected beam of light serves as a weightless pointer, and enables us to measure the deflection of the object which carries the mirror. (6) When a beam of light is reflected at each of two mirrors, inclined at a given angle, the ultimate deviation of the beam is (if the whole path of the light be within one plane) equal to twice the angle between the mirrors. This proposition is applied in the quadrant and (7) When a wave of any form is reflected at a plane surface it retains after reflection the form which it would have assumed but for the reflection, this form being, however, guided by reflection into a different direction.
Curved Reflecting Surfaces. In these we have to trace out the mode of reflection of incident rays from each "element" or little bit of the reflecting surface; and this leads, through geometrical working, to such propositions as the following: (1) Parallel rays, traveling parallel to the axis of a concave paraboloid mirror are made to converge so as all actually to pass accurately through the geometrical focus of the paraboloid; and, conversely, if the source of light be at the geometrical focus, the rays re flected from the mirror emerge parallel REFLECTION το one another-a proposition of great utility in lighthouse work, search-lights, etc. (2) If the paraboloid mirror be convex, parallel incident rays have, after reflection, the same course as if they had come from the geometrical focus of the paraboloid. (3) In a concave ellipsoid mirror, light diverging from one "focus" of the ellipsoid is reflected so as to converge on the other "focus" of the curved verge on the other "focus" of the curved surface; and by a convex ellipsoidal mirror light converging toward the one focus is made to diverge as if it had come directly from the other focus. In hyperboloid reflector the two geometrical foci have properties corresponding to those of the ellipsoid. (5) In spherical reflectors, which are those most easily made, there is no accurate focus except for rays proceeding from the center and returning to it. When parallel rays are incident on a concave spherical mirror we see that if they be parallel to the axis of the mirror each ray is made to pass after reflection through a point, which is nearer to a point midway between the mirror and its center, the narrower is the pencil of rays. If therefore, the pencil of rays be very narrow in comparison with the radius, the rays will, after reflection, approximately converge on the midway point, int, which is called the principal focus of the mirror.
The reflected rays from the various parts of the mirror form by their intersection at the midway point.
As to the quality of the light reflected there are some peculiarities to be observed. From the surface of a transparent body, of greater optical density than the surrounding medium, light polarized in the plane of incidence and reflection is more largely reflected at oblique incidences than light polarized at right angles to that plane; when the angle of incidence is such that the reoblique incidences than light polarized right angles to one another, the whole of the light reflected is polarized in the plane incidence and reflection; and if light polarized at right angles to that plane be made to fall on glass at the particular angle of incidence just referred to, it will not be reflected at all, but will w wholly enter the glass. Planepolarized light polarized in any other plane than that of incidence or one at right angles to it, is, after total reflecferred to, it will not be reflected at all, but will wholly enter the glass. Planelarized; and this phenomenon is always presented in reflection from metals. In the case of electro-magnetic radiation theory and practice concur in indicating that conductors are good while non-conpresented in reflection from metals. In the case of electro-magnetic radiation erence to those more frequent but other- wise similar ether oscillations to which erence to those more frequent but otherwise similar ether oscillations to which the phenomena of radiant heat, light, and actinism are due.