Infrared and ultraviolet radiation

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Infrared and ultraviolet radiation If a spectrum from an electric arc lamp or from the sun is produced on a screen by one of the methods described in this chapter it may be shown that invisible radiant energy is incident on the screen just beyond each of the extreme ends of the visible spectrum (Fig. 25.7). Just beyond […]

Mixing coloured pigments

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Mixing coloured pigments One of the first things that a student of painting learns is that green paint can be made by mixing yellow and blue paint. This would not be possible if the paints available were pure yellow and pure blue. The success of this method of making green paint depends on the fact that the pigments […]

Mixture of coloured lights

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Mixture of coloured lights Before proceeding further it must be pointed out that mixing coloured paints is an entirely different thing from mixing coloured lights. Paints will be dealt with later. The effect of mixing coloured lights may be investigated by using three projectors fitted with slides of various coloured gelatine sheets, and arranged so as to produce overlapping […]

Primary and secondary colours

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Primary and secondary colours Although yellow may be produced by mixing red and green lights, it is not found possible to produce either red, green, or blue by mixing two other colours. For this reason, red, green, and blue are called primary colours. Yellow is called a secondary colour. The other two secondary colours are cyan made by […]

Appearance of coloured objects in coloured light

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Appearance of coloured objects in coloured light A convenient source of light of various colours may be obtained by placing an ordinary electric lamp in a box with an opening which may be covered with gelatine sheets of different colours. By means of such a lamp the appearance of different coloured objects in different colours of light […]

Light filters

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Light filters Interesting results are obtained when light filters in the form of sheets of gelatine coloured with various dyes are placed, in turn, in front of the slit in Fig. 25.2 or 25.3. By this means, the light transmitted by the filter can be analysed into its component colours. It is observed that certain colours, depending […]

Colour of objects in white light

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Colour of objects in white light When white light falls on any particular body, then either all the colours in the white light may be reflected from the body, when it appears white, or only some of them may be reflected while the others are absorbed. In the latter case the body appears coloured. The energy of the […]

Recombination of the colours of the spectrum

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Recombination of the colours of the spectrum The colours of the spectrum may be recombined to form white light by aIlowing the spectrum to be formed on a row of smaIl rectangular plane mirrors (Fig. 25.5). On adjusting the angle which the mirrors make with the incident light so that they all reflect the light to the […]

Production of a pure spectrum. Spectrometer

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Production of a pure spectrum. Spectrometer A better method of obtaining a pure spectrum than the one just described is to use a parallel beam of light as in an instrument called a spectrometer. This is used for examining the spectra of hot gases and other sources of light (Fig. 25.3). An illuminated slit is placed at […]

Improvement on Newton’s original experiment

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Improvement on Newton’s original experiment The spectrum formed in Newton’s first experiment was impure. This resulted from the fact that it consisted of a series of circular coloured images all overlapping. Later Newton devised the arrangement shown in Fig. 25.2, which produced a fairly pure spectrum. A converging lens L is placed so as to form an […]