Motion Of Light

Rays of light are thrown off from luminous bodies in every direction, but always in straight lines, which cross each other at every point; but the particles of which each ray consists are so minute that the rays do not appear to be impeded by each other. A ray of light passing through an aperture into a dark room, proceeds in a straight line; a fact of which any one may be convinced by going into a darkened room and admiting light only through a small aperture.

Light also moves with great velocity, but becomes fainter as it recedes from the source from which it eminates; in other words, diverging rays of light diminish in intensity as the square of the distance increases. For instance let a fig. 1, represent the luminous body from which light proceeds, and suppose three square boards, b. c. d. severally one, four and sixteen square inches in size be placed; b one foot, c two feet, and d four feet from a, it will be perceived that the smallest board b will throw c into shadow; that is, obstruct all rays of light that would otherwise fall on c, and if b were removed c would in like manner hide the light from d--Now, if b recieve as much light as would fall on c whose surface is four times as large, the light must be four times as powerful and sixteen times as powerful as that which would fall on the second and third boards, because the same quantity of light is diffused over a space four and sixteen times greater. These same rays may be collected and their intensity again increased.

Rays of light are reflected from one surface to another; Refracted, or bent, as they pass from the surface of one transparent medium to another; and Inflected, or turned from their course, by the attraction of opaque bodies. From the first we derive the principles on which mirrors are constructed; to the second we are indebted for the power of the lenses, and the blessings of sight,--for the light acts upon the retina of the eye in the same manner as on the lens of a camera. The latter has no important bearing upon our subject.

When a ray of light falls perpendicularly upon an opaque body, it is reflected bark in the same line in which it proceeds; in this case the reflected ray returns in the same path the incident ray traversed; but when a ray falls obliquely, it is reflected obliquely, that is, it is thrown off in opposite direction, and as far from the perpendicular as was the incident ray, as shown at Fig. 2; a representing the incident ray and b the reflected. The point, or angle c made by the incident ray, at the surface of the reflector e f, with a line c d, perpendicular to that surface, is called the angle of incidence, while the angle formed by the reflected ray b and the perpendicular line d is called the angle of reflection, and these angles are always equal.

It is by this reflection of light that objects are made visible; but unless light falls directly upon the eye they are invisible, and are not sensibly felt until after a certain series of operations upon the various coverings and humors of the eye. Smooth and polished surfaces reflect light most powerfully, and send to the eye the images of the objects from which the light proceeded before reflection. Glass, which is transparent-- transmitting light--would be of no use to us as a mirror, were it not first coated on one side with a metalic amalgam, which interrupts the rays in their passage from the glass into the air, and throws them either directly in the incident line, or in an oblique direction. The reason why trees, rocks and animals are not all mirrors, reflecting other forms instead of their own, is, that their surfaces are uneven, and rays of light reflected from an uneven surface are diffused in all directions.

Parallel rays falling obliquely upon a plane mirror are reflected parallel; converging rays, with the same degree of convergence; and diverging rays equally divergent.

Stand before a mirror and your image is formed therein, and appears to be as far behind the glass as you are before it, making the angle of reflection equal to that of incidence, as before stated. The incident ray and the reflected ray form, together, what is called the passage of reflection, and this will therefore make the actual distance of an image to appear as far again from the eye as it really is. Any object which reflects light is called a radiant. The point behind a reflecting surface, from which they appear to diverge, is called the virtual focus.

Rays of light being reflected at the same angle at which they fall upon a mirror, two persons can stand in such a position that each can see the image of the other without seeing his own. Again; you may see your whole figure in a mirror half your length, but if you stand before one a few inches shorter the whole cannot be reflected, as the incident ray which passes from your feet into the mirror in the former case, will in the latter fall under it. Images are always reversed in mirrors.

Convex mirrors reflect light from a rounded surface and disperse the rays in every direction, causing parallel rays to diverge, diverging rays to diverge more, and converging rays to converge less-- They represent objects smaller than they really are--because the angle formed by the reflected ray is rendered more acute by a convex than by a plane surface, and it is the diminishing of the visual angle, by causing rays of light to be farther extended before they meet in a point, which produces the image of convex mirrors. The greater the convexity of a mirror, the more will the images of the objects be diminished, and the nearer will they appear to the surface. These mirrors furnish science with many curious and pleasing facts.

Concave mirrors are the reverse of convex; the latter being rounded outwards, the former hollowed inwards--they render rays of light more converging-- collect rays instead of dispersing them, and magnify objects while the convex diminishes them.

Rays of light may be collected in the focus of a mirror to such intensity as to melt metals. The ordinary burning glass is an illustration of this fact; although the rays of light are refracted, or passed through the glass and concentrated into a focus beneath.

When incident rays are parallel, the reflected rays converge to a focus, but when the incident rays proceed from a focus, or are divergent, they are reflected parallel. It is only when an object is nearer to a concave mirror than its centre of concavity, that its image is magnified; for when the object is farther from the mirror, this centre will appear less than the object, and in an inverted position.

The centre of concavity in a concave mirror, is an imaginary point placed in the centre of a circle formed by continuing the boundary of the concavity of the mirror from any one point of the edge to another parallel to and beneath it.