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Light and Colour

colours, spectrum, regions, green, propagation, velocity and radiations

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LIGHT AND COLOUR x. White Light. When a beam of daylight, which we call white light, in spite of the fact that its quality is continually changing,' is decomposed, e.g. by passing it through a glass prism which separates it into the constituent elements of the incident light, the colours of the rainbow may be seen in their normal order. By using suitable apparatus, described in all books on optics, a pure spectrum, can be obtained consisting of an infinite number of images of a fine slit, each image being formed by one of the elementary radiations which go to make up the light by which the slit is illuminated.

Though there are an infinite number of colours in the spectrum, it is usual to divide them into groups, in each of which the eye experiences sensations which differ little from one another. For reasons as arbitrary as those which formerly caused the number of days in the week, or the wonders of the world to be fixed at seven, the number of spectral colours has also been fixed at seven, and a bad alexandrine of the Abbe Belisle— Violet, Indigo, Blue, Green, Yellow, Orange, and Red has contributed quite appreciably to keeping alive this unfortunate tradition, from which most exaggerated conclusions have often been drawn.

Examination of the colours of the rainbow or of a slightly dispersed normal shows that the spectrum may be divided into three chief regions of equal extent, the blue-violet, green, and vermilion-red, in each of which the variation of tint is almost imperceptible. Be tween these large regions there are narrow tran sition regions where the variation of colour is very rapid, one being a blue-greenish colour be tween the blue-violet and the green ; the other, yellow between the green and the red. For most practical purposes it is sufficient to consider the three main regions into which the spectrum may, roughly speaking, be equally divided. Thus, if the spectrum is to be considered as divided into more than five regions (the three principal and the two transition regions), then one ought to speak of an infinite number of colours rather than only seven.

2. LightWaves—Wave-length. As the colours are infinite in number, obviously it is impossible to name them all accurately by words, and it is necessary to use numbers. Spectral radiations

are universally specified by their wave-lengths.

This numeration of colours, independent of any arbitrary convention, was made possible when the researches of Young and Fresnel proved that light is the propagation of a periodic vibration, which may be compared (provided the comparison is not pushed too far) to the propagation of sound (sound waves), or to the propagation of waves created on the surface of smooth water by the falling of a stone, or even to the course of a person walking in a straight line with a uniform velocity and a regular step. Without attempting here any justification of this wave theory, one may mention, however, amongst the numerous facts which may he quoted in support of it, the coloration of thin films (soap bubbles) and the direct process of colour photography due to G. Lippmann (inter ference method).

In all cases of the propagation of a periodic phenomenon there are three quantities involved : the velocity of propagation, the time of one period (or its reciprocal, the frequency), and the " step " or wave-length, which are connected by the relations Wave-length = velocity x time of one period velocity — frequency* The velocity of light in air is approximately 187,000 miles per second, and is the same for all radiations.

While the pitch of a note is always character ized by its frequency, radiations are always denoted by their wave-length (sometimes repre sented by the Greek letter 2, lambda), which is who are colour-blind cannot perceive certain colours ; they generally confuse red and green), the region which appears the most luminous in the normal spectrum of white light is the green. At high luminous intensities the maximum of luminosity is at 5,50o A.U. (yellow-green) ; at very weak illuminations the red, which normally is more luminous than the blue, begins to fade sooner than the blue, the sensation of colour disappears almost completely, and the maximum of luminosity approaches continually closer to 5,300 A.U. (blue-green). This effect, known as the Purkinie phenomenon, is, as we shall see the distance from crest to crest of two successive waves, measured generally in ten-millionths of a millimetre, that is, in Angstrom units (A.U.).

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