REFLECTION

, or Reflexion, in Mechanics, is the return, or regressive motion of a moveable body, occasioned by the resistance of another body, which hinders it from pursuing its former course of direction.

Reflection is conceived, by the latest and best authors, as a motion peculiar to elastic bodies, by which, after striking on others which they cannot remove, they recede, or turn back, or aside, by their elastic power.

On this principle it is asserted, that there may be, and is, a period of rest between the incidence and the reflection; since the reflected motion is not a continuation of the other, but a new motion, arising from a new cause or principle, viz, the power of elasticity.

It is one of the great laws of Reflection, that the angle of incidence is equal to the angle of Reflection; i. e. that the angle which the direction of motion of a striking body makes with the surface of the body struck, is equal to the angle made between the same surface and the direction of motion after the stroke. See Incidence and Percussion.

Reflection of the Rays of Light, like that of other bodies, is their motion after being reflected from the surfaces of bodies.

The Reflection of the rays of light from the surfaces of bodies, is the means by which those bodies become visible. And the disposition of bodies to reflect this or that kind of rays most copiously, is the cause of their being of this or that colour. Also, the Reflection of light, from the surfaces of mirrors, makes the subject of catoptrics.

The Reflection of light, Newton has shewn, is not effected by the rays striking on the very parts of the bodies; but by some power of the body equally diffused throughout its whole surface, by which it acts upon the ray, attracting or repelling it without any real immediate contact. This power he also shews is the same by which, in other circumstances, the rays are refracted; and by which they are at first emitted from the lucid body.

Dr. Priestley says, it is not more probable, that the rays of light are transmitted from the sun, with an uniform disposition to be reflected or refracted, according to the circumstances of the bodies on which they impinge; and that the transmission of some of the rays, apparently under the same circumstances, with others that are reflected, is owing to the minute vibrations of the small parts of the surfaces of the mediums through which the rays pass; vibrations that are independent of action and reaction between the bodies and the particles of light at the time of their impinging, though probably excited by the action of preceding rays. Hist. of Light and Colours, pa. 309.

Newton concludes his account of the Reflection of light with observing, that if light be reflected not by impinging on the solid parts of bodies, but by some other principle, it is probable that as many of its | rays as impinge on the solid parts of bodies are not reflected, but stifled and lost in the bodies. Otherwise, he says, we must suppose two kinds of Reflection; for should all the rays be reflected which impinge on the internal parts of clear water or crystal, those substances would rather have a cloudy colour, than a clear transparency. To make bodies look black, it is necessary that many rays be stopped, retained and lost in them; and it does not seem probable that any rays can be stopped and stifled in them, which do not impinge on their parts: and hence, he says, we may understand, that bodies are much more rare and porous than is commonly believed. However, M. Bouguer disputes the fact of light being stifled or lost by impinging on the solid parts of bodies.

, in Catoptrics, is the return of a ray of light from the polished surface of a speculum or mirror, as driven thence by some power residing in it.

The ray thus returned is called a reflex or reflected ray, or a ray of Reflection; and the point of the speculum where the ray commences, is called the point of Reflection. Thus, the ray AB, proceeding from the radiant A, and striking on the point of the speculum B, being returned thence to C, BC represents the reflected ray, and B the point of Reflection; in respect of which, AB represents the incident ray, or ray of incidence, and B the point of incidence; also the angle CBE is the angle of Reflection, and ABD the angle of incidence; where DE is the reflecting surface, or at least a tangent to it at the point B. Though some count the angle of incidence and of Reflection from the perpendicular BF.

General Laws of Reflection.—1. When a ray of light is reflected from a speculum of any form, the angle of incidence is always equal to the angle of Reflection. This law obtains in the percussions of all kinds of bodies; and consequently must do so in those of light; and the proof of it may be seen at the article INCIDENCE.

This law is confirmed also by experiments on all bodies; and on the rays of light in this manner: A ray from the sun falling on a mirror, in a dark room, through a small hole, you will have the pleasure to see it rebound, so as to make the angle of Reflection equal to the angle of incidence. And the same may be shewn in various other ways; thus ex. gr. placing a semicircle DFE on a mirror DE, its centre on B, and its limb or plane perpendicular to the speculum; and assuming equal arcs DG and EH; place an object in A, and the eye in C: then will the object be seen by a ray reflected from the point B. But by covering B, the object will cease to be seen.

II. Every point of a speculum reflects rays falling on it, from every part of an object.

III. If the eye C and the radiant point A change places, the point will continue to radiate upon the eye, in the Jame course or path as before.

IV. The plane of Reflection is perpendicular to the surface of the speculum; and it passes through the centre in spherical specula.

Reflection of the Moon, is a term used by some authors for what is otherwise called her variation; being the 3d inequality in her motion, by which her true place out of the quadratures differs from her place twice equated.

Reflection is also used in the Copernican system, for the distance of the pole from the horizon of the disc; which is the same thing as the sun's declination in the Ptolomaic system.

REFLECTOIRE Curve. See Reflectoire Curve.

REFLEXIBILITY of the rays of light, is that property by which they are disposed to be reflected. Or, it is their disposition to be turned back into the same medium, from any other medium on whose surface they fall. Hence those rays are said to be more or less reflexible, which are returned back more or less easily under the same incidence. Thus, if light pass out of glass into air, and by being inclined more and more to the common surface of the glass and air, begins at length to be totally reflected by that surface, those sorts of rays which at like incidences are reflected most copiously, or the rays which by being inclined begin soonest to be totally reflected, are the most reflexible rays.

That rays of light are of different colours, and endued with different degrees of reflexibility, was first discovered by Sir I. Newton; and it is shewn by the following experiment. Applying a prism DFE to the aperture C of a darkened room in such manner that the light be reflected from the base in G; the violet rays are seen first reflected into HG; the other rays continuing still refracted to I and K. After the violet, the blue are all reflected; then the green, &c.—Hence it appears, that the differently coloured rays differ in degree of Reflexibility. And from other experiments it appears that those rays which are most reflexible, are also most refrangible.

REFLUX of the Sea, is the ebbing of the water, or its return from the shore; being so called, because it is the opposite motion to the flood or flux. See Tide.

REFRACTED Angle, or Angle of Refraction, in Optics, is the angle which the refracted ray makes with the refracting surface; or sometimes it denotes the complement of that, or the angle it makes with the perpendicular to the said surface.

Refracted Dials, or Refracting Dials, are such as shew the hour by means of some refracting transparent fluid.

Refracted Ray, or Ray of Refraction, is a | ray after it is broken or bent, at the common surface of two different mediums, where it passes from the one into the other. See Ray, and REFRACTION.