I.—The angle of reflection is equal to the angle of incidence.
incident and the reflected ray are both in the same plane, which is perpendicular to the reflecting surface.
All the phenomena of reflection of rays of from rullichpri ctirfanAc mhAtli rs1.110 curved, take place in accordance with these two laws. If the polished surface be a plane, or have a regular curvature, the reflected rays of light produce images of the objects from which the rays have proceeded. If the surface be roughened, the reflection will still be pro duced, but the rays are irregularly dispersed, and no images are produced. By the term reflecting surfaces is usually meant polished surfaces, but even when a ray of light meets a perfectly polished surface only some of it is reflected. A portion of it is absorbed by the medium forming the substance of the polished surface, another part is diffused, and another is transmitted, if the medium be transparent.
Let A B (Fig. 3o9) be a plane reflecting surface, and R D a ray of light incident upon it, the ray will be reflected in the direction of D R' in the same plane as the incident ray, the plane being perpendicular to the reflecting surface, and making with that surface an angle R' D B equal to the angle R D A. The normal C D is the straight line perpendicular to the surface A B, which makes the incident ray R D and the reflected ray R' D equal angles.
Bodies having polished surfaces are termed mirrors, and show by reflection images of objects presented to them. The place at which objects appear is their image. The quantity of light reflected from the surface of a mirror depends, firstly, upon the degree of perfection of its polish; secondly, on the color of its surface; and thirdly, upon the incidence of the luminous rays which strike it. Mirrors are divided, accord ing to their form, into the following: Plane, concave, spherical, parabolic, conical, &c.
The commonest example of the plane mirror is the ordinary looking-glass. Its effect is shown in Fig. 31o. The eye not only sees the object, but also a reflection of it, be cause the light that emanates from the ball in the direction of the line towards the mirror has its direction changed and enters the eye. If light from the object reaches the eye in two
distinct directions the effect is precisely the same as if there were two objects, because two images are produced upon the retina. This reflection, which, although it appears like an image, is not one, is termed in optics a virtual image. When the reflected rays converge, however, as is the case with concave mirrors, the rays coincide at a point in front of the mirror on the same side as the object. They then form an image termed a real image that can be received on a screen.
Polished metal mirrors give only one image ; glass mirrors give several, however. If a candle be placed near a looking glass, and its reflection received obliquely, a feeble image is first seen in front of the principal and distinct one. Behind this several other images are observed, the intensities of which gradually diminish as they disappear. This phenomenon is due to the fact of the looking-glass possessing two reflecting surfaces.
Oblique rays are partly reflected from the front surface of the glass which gives the faint image first mentioned, in addition to the principal image reflected from the back surface. The other images are produced by the light from the back, or prepared surface, being reflected back again from the front surface of the glass, and so going back wards and forwards, producing numbers of images which naturally get fainter and fainter. The passage of the ray is more clearly shown in Fig. 311. The thick line indicates the passage of the light ray, and the dotted lines show the reflections producing the false images.
It is for the above reason that glass mirrors cannot be used for many purposes. For instance, in the making of reversed negatives, metal mirrors will be found preferable.
We have already pointed out that, besides plane mirrors, there are several other kinds ; those now frequently used are the spherical and parabolic reflectors.