However, when rays travel at an angle relative to the original direction of the beam, each ray travels a different distance to a common location, and they can arrive in or out of phase. When they travel straight ahead, as in part (a) of the figure, they remain in phase, and we observe a central maximum. (Each ray is perpendicular to the wave front of a wavelet.) Assuming the screen is very far away compared with the size of the slit, rays heading toward a common destination are nearly parallel. These are like rays that start out in phase and head in all directions. According to Huygens’s principle, every part of the wave front in the slit emits wavelets, as we discussed in The Nature of Light. We then consider light propagating onwards from different parts of the same slit. Here, the light arrives at the slit, illuminating it uniformly and is in phase across its width. The analysis of single-slit diffraction is illustrated in (Figure). (b) The diagram shows the bright central maximum, and the dimmer and thinner maxima on either side. The central maximum is six times higher than shown. (a) Monochromatic light passing through a single slit has a central maximum and many smaller and dimmer maxima on either side.
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