Light

LIGHT, subjectively, the sense impression formed in the eye. (See VisioN.) The present article deals with it purely objectively and is concerned with the more fundamental characteristics of light and optical instruments. For the more practical applications, which are not here discussed in detail, see OPTICS; TELESCOPE; MICROSCOPE ; INTERFEROMETER ; PHOTOMETRY. The subject is conveniently still further subdivided according as to whether we are more interested in how the light originates or how it behaves after it has been emitted. The first subject is treated under RADIATION, THEORY OF ; and SPECTROSCOPY ; the present article is chiefly concerned with the behaviour of light after it has been emitted, a branch of the subject often called Physical Optics, dealing almost entirely with the Wave Theory of Light.

It might perhaps be expected that we should begin by saying what light "really" is, and should then develop its characters from such a starting point ; but this procedure is not possible, since light is essentially more primitive than any of the things in terms of which we might try to explain it. The nature of light is only describable by enumerating its properties and founding them on the simplest possible principles. As these principles transcend our ordinary experiences they must be cast in a purely logical, that is to say mathematical, form. But that is never enough, for, though logic tells us what deductions must be right, it does not tell us what will be interesting, and so gives no guidance as to the direc tion the theory will take. In choosing this direction much help is derived from analogies and models, which are often loose and incomplete, but without which no proper understanding of the subject can be acquired. We shall therefore describe, largely by means of analogies, the behaviour of light, and this is the "real" nature of light.

Types of Radiation.

Light would be taken strictly speaking It is not convenient to use the same units throughout such an enormous range. It is customary to measure wireless waves in metres. The "rest-rays" are often measured in units called the thousandth of a millimetre, but for light the unit universally used is the Angstrom Unit (A.U.) which is cm. This is also most frequently used for X-rays, though in precision work X-units of cm. have been introduced. Much of what we shall say here applies to the whole range, but it is not unnatural that there should be differences, and when these occur we shall restrict our selves to the case of light, visible, ultra-violet and infra-red.

The Appearance of Ordinary Things.

In many branches of science, though we start with familiar and crude observations, we very soon find it necessary to discuss much more recondite questions, and this leads to an elaboration of the theory, until the most important things in it are quite different from what is f a miliar to us. This is very emphatically true of light. For example, the most familiar thing about light is that it goes in straight lines, and hardly any of our ordinary experiences disagree with this fact. But by more refined experiments it is easy to exhibit that light is sometimes bent (diffracted) and such cases prove more important to the theory than the commonplace observations. Al most the only example of these phenomena, diffraction and inter ference, which is a matter of common experience, is the iridescent colours that appear on soap-bubbles and on the surface of greasy water.

So it comes about that the theory of light is not much con cerned with the appearance of ordinary things, and, as we shall later have very little to say about them, we may consider them briefly here. In things we see, we may distinguish between the luminous and the non-luminous—in day-light between the sun and everything else. The non-luminous objects are only visible by light reflected from some luminous source, the sun or a lamp, and their different appearances are solely due to the different ways in which they reflect light. Thus a black object is one which does not reflect at all. A coloured object is one which reflects some colours but not others. This is easily proved by illuminating a piece of red paper by light passed through a green glass, when it will appear black because there was no red light for it to reflect. White being a mixture of all the colours, a white object is one which reflects all colours about equally, and so it looks coloured if lit by a coloured light. When two pigments are mixed together the resulting colour will be that which they are both capable of reflecting, and may be quite different from the colour seen when the light of two coloured lamps is projected together on to a white screen; for example if the light from red and green lamps is compounded in this way the result is a brilliant yellow, but red and green paints when mixed give a dark muddy colour because there is little light which they can both reflect (see CoLouR).