The science of light . darkness atQ and at a point midway between P and Q. The brightband will therefore be narrower, and the dark bandbroader, than with two slits. If we represent graphicallythe brightness at different distances from P we shall geta curve something like Fig. 40, where the thick solid linerepresents the brightness with two slits, and the dotted DIFFRACTION 69 line with four slits. We see that there are two smallsubsidiary maxima with the four slits, and these ofcourse appear as faint bands in between the brightmain bands. If we have eight equidistant slits we candivide them in
The science of light . darkness atQ and at a point midway between P and Q. The brightband will therefore be narrower, and the dark bandbroader, than with two slits. If we represent graphicallythe brightness at different distances from P we shall geta curve something like Fig. 40, where the thick solid linerepresents the brightness with two slits, and the dotted DIFFRACTION 69 line with four slits. We see that there are two smallsubsidiary maxima with the four slits, and these ofcourse appear as faint bands in between the brightmain bands. If we have eight equidistant slits we candivide them into pairs, 1 and 5, 2 and 6, 3 and 7, 4 and 8,which are four times as far apart as AB, and these willtherefore produce darkness at a point only a quarter asfar as Q from P. The intensity of the central bandtherefore falls off to zero very rapidly, as shown in thethin solid line in Fig. 40. It will be noticed that wehave six subsidiary maxima in between the mainmaxima, which, if visible at all, will appear as very faint. (Distance from P Fig. 40. bands in between the bright ones. Every increase inthe number of equidistant slits leads to a narrowing ofthe bright band and a broadening of the dark space inbetween, until when the number of slits is very largethe bright bands are sharp narrow lines separated bybroad dark spaces. The distances of these lines fromthe centre will depend upon the wave-length of thelight, being greater for the greater wave-lengths. Eachcolour will therefore produce a sharp narrow line, andwe shall get a pure spectrum without any overlappingof the colours. The Diffraction Grating.—The diffraction grating ismerely a large number of parallel equidistant slits. Arough grating is provided by a piece of fine wire gauze,or even by a thin plain silk handkerchief. Lookingthrough the grating at an illuminated slit, or a gas flam; 70 THE SCIENCE OF LIGHT turned edgewise, we see on each side of the slit or flamecoloured images which are violet on the inside and redo
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