The science of light . pinholes in Youngsexperiment are replaced by parallel slits, otherwise theexperiments are exactly similar. It is evident, there-fore, that we shall get a series of dark and light bandsparallel to the slits. Suppose A and B, in Fig. 39, arethe two slits placed close together, and that the rays of 68 THE SCIENCE OF LIGHT light are coming up in the direction of the arrows fromthe slit some distance away. If the point P is chosen on the screen, so that AP = BP,the wavelets emerging from A and B will reach P in thesame phase, and will therefore reinforce one another andform a
The science of light . pinholes in Youngsexperiment are replaced by parallel slits, otherwise theexperiments are exactly similar. It is evident, there-fore, that we shall get a series of dark and light bandsparallel to the slits. Suppose A and B, in Fig. 39, arethe two slits placed close together, and that the rays of 68 THE SCIENCE OF LIGHT light are coming up in the direction of the arrows fromthe slit some distance away. If the point P is chosen on the screen, so that AP = BP,the wavelets emerging from A and B will reach P in thesame phase, and will therefore reinforce one another andform a bright band. If Q is chosen, so that AQ is justhalf a wave-length longer than BQ, then the waveletsemerging from A and B will reach Q in opposite phases,and therefore we shall get darkness. As we move outfrom P to Q the two wavelets will be getting more andmore out of step until this opposition of phase is shall therefore get a gradual shading off from themaximum brightness at P to the complete darkness at. Fig. 39. Q. This gradual shading off is characteristic of all thebands produced by interference or diffraction which wehave considered up to the present. Now suppose wehave four slits equally spaced instead of two. We shallget agreement of phase at P for all four slits and opposi-tion of phase at Q for the two pairs of slits, 1 and 2, and3 and 4. But at a point halfway between P and Q weshall also get opposition of phase between the two pairs,1 and 3, and 2 and 4, for they are twice as far apart asthe first two pairs. We shall therefore get 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 wit
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