RAY

, in Geometry, the same as Radius.

, in Optics, a beam or line of light, propagated from a radiant point, through any medium.

If the parts of a Ray of light lie all in a straight line between the radiant point and the eye, the Ray is said to be direct: the laws and properties of which make the subject of Optics.—If any of them be turned out of that direction, or bent in their passage, the Ray is said to be refracted.—If it strike on the surface of any body, and be thrown off again, it is said to be reflected.—In each case, the Ray, as it falls either directly on the eye, or on the point of reflection, or of refraction, is said to be incident.

Again, if several Rays be propagated from the radiant object equidistantly from one another, they are called parallel Rays. If they come inclining towards each other, they are called converging Rays. And if they go continually receding from each other, they are called diverging Rays.

It is from the different circumstances of Rays, that the several kinds of bodies are distinguished in Optics. A body, for example, that diffuses its own light, or emits Rays of its own, is called a radiating or lucid or luminous body. If it only reflect Rays which it receives from another, it is called an illuminated body. If it only transmit Rays, it is called a transparent or translucent body. If it intercept the Rays, or refuse them passage, it is called an opaque body.

It is by means of Rays reflected from the several points of illuminated objects to the eye, that they become visible, and that vision is performed; whence such Rays are called visual Rays.

The Rays of light are not homogeneous, or similar, but differ in all the properties we know of; viz, refrangibility, reflexibility, and colour. It is probably from the different refrangibility that the other differences have their rise; at least it appears that those Rays which agree or differ in this, do so in all the rest. It is not however to be understood that the property or effect called colour, exists in the Rays of light themselves; but from the different sensations the differently disposed Rays excite in us, we call them red Rays, yellow Rays, &c. Each beam of light however, as it comes from the sun, seems to be compounded of all the sorts of Rays mixed together; and it is only by splitting or separating the parts of it, that these different sorts become observable; and this is done by transmitting the | beam through a glass prism, which refracting it in the passage, and the parts that excite the different colours having different degrees of refrangibility, they are thus separated from one another, and exhibited each apart, and appearing of the different colours.

Beside refrangibility, and the other properties of the Rays of light already ascertained by observation and experiment, Sir I. Newton suspects they may have many more; particularly a power of being inflected or bent by the action of distant bodies; and those Rays which differ in refrangibility, he conceives likewise to differ in flexibility.

These Rays he suspects may be very small bodies emitted from shining substances. Such bodies may have all the conditions of light: and there is that action and reaction between transparent bodies and light, which very much resembles the attractive force between other bodies. Nothing more is required for the production of all the various colours, and all the degrees of refrangibility, but that the Rays of light be bodies of different sizes; the least of which may make violet the weakest and darkest of the colours, and be the most easily diverted by refracting surfaces from its rectilinear course; and the rest, as they are larger and larger, may make the stronger and more lucid colours, blue, green, yellow, and red. See Colour, Light, REFRACTION, Reflection, Inflection, Converging, Diverging, &c, &c.

Reflected Rays, those Rays of light which are reflected, or thrown back again, from the surfaces of bodies upon which they strike. It is sound that, in all the Rays of light, the angle of reflection is equal to the angle of incidence.

Refracted Rays, are those Rays of light, which are bent or broken, in passing out of one medium into another.

Pencil of Rays, a number of Rays issued from a point of an object, and diverging in the form of a cone.

Principal Ray, in Perspective, is the perpendicular distance between the eye and the vertical plane or table, as some call it.

Ray of Curvature. See Radius of Curvature.