Reflection

Reflection. Radiation falling upon any bright surface is either partially or totally reflected, according to the nature of the surface. It will be simplest to consider the reflection of that particular form of radiation which we call light. If we let a beam of light fall upon a polished surface - say, a plane mirror - we shall note that the reflected beam , and incident beam are in the same plane with the normal to the mirror at the point of incidence, and both beams make the same angle with this normal. If the mirror be perfectly smooth and bright, it would only be able to reflect light in this regular manner; hence it would give us images of other bodies, but would itself be invisible. Bodies, however, do not usually reach this state of perfect polish, but irregularities in the surface cause the light to be irregularly reflected. Every incident ray is reflected according to the above laws, but differences in inclination of a multitude of small surfaces making up the whole, produce the effect of the light being reflected in all directions. This is what is known as diffusion of light, and it is in virtue of this that a body becomes visible. A plane mirror produces an image behind it exactly similar in size and shape to the object in front. Thus, let A B be an object in front of h, mirror, M m' (Fig. 1). A pencil of rays, A P, starting from A, is reflected in the direction P E, and appears to the eye (E) of an observer as though it came from a point a'.

Similarly the pencil b

is reflected so that the reflected rays seem to start from b' f and every point in A b similarly sends out beams of light, the result being the formation of the image a' b', which is virtual, since the rays p e, etc., do not actually pass through it. It is easy to show that a b and A' b' are equal and equi-distant from M si'. An image formed by a plane mirror is not a facsimile of the object; thus an image of the page of a book bears to the page itself the same relation as the type does to the actual print. If two mirrors be placed parallel to each other, an infinite number of images will be formed of an object between them, and a symmetrical arrangement of images is always formed when the angle between two mirrors is an aliquot part of 360°. This principle is employed in the kaleidoscope (q.v.) Mirrors are often made spherical in shape, and in this case an image is formed which is usually not so sharply defined as in the case of a plane mirror. This is due to snherical aberration (q.v.). In the case of a concave mirror, when the object (o) is farther away from the mirror than the centre of curvature (c), the image (i) is smaller than the object, and is formed between c and a point (v) half-way between c and the mirror (Fig. 2). As the object moves towards C, the image, which is real and inverted, moves to meet it, so that both coincide at c. As an illustration of this, a man may so arrange matters that his hand may appear to be shaking its own image. If the object be between F and the mirror, the image is behind the mirror, is erect and virtual. If an object, ab, be placed in front of a convex mirror, the image A' b' is behind the mirror, and is virtual and erect. This is shown in the accompanying figure, and the construction is equally easy for the cases quoted previously with regard to a concave mirror. Concave mirrors are often used for medical purposes, since light can be concentrated on any special object by means of them. They are also used in reflecting telescopes. Convex mirrors are not often employed.