One ray travels a distance different from the ray from the bottom and arrives in phase, interfering constructively. The difference in path length for rays from either side of the slit is seen to be D sin θ.Īt the larger angle shown in Figure 2(c), the path lengths differ by for rays from the top and bottom of the slit. Light passing through a single slit is diffracted in all directions and may interfere constructively or destructively, depending on the angle. There will be another minimum at the same angle to the right of the incident direction of the light. In fact, each ray from the slit will have another to interfere destructively, and a minimum in intensity will occur at this angle. A ray from slightly above the center and one from slightly above the bottom will also cancel one another. Thus a ray from the center travels a distance farther than the one on the left, arrives out of phase, and interferes destructively. In Figure 2(b), the ray from the bottom travels a distance of one wavelength farther than the ray from the top. However, when rays travel at an angle relative to the original direction of the beam, each travels a different distance to a common location, and they can arrive in or out of phase.
When they travel straight ahead, as in Figure 2(a), they remain in phase, and a central maximum is obtained. (Each ray is perpendicular to the wavefront 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 wavefront in the slit emits wavelets. Here we consider light coming from different parts of the same slit. The analysis of single slit diffraction is illustrated in Figure 2. (b) The drawing shows the bright central maximum and dimmer and thinner maxima on either side. The central maximum is six times higher than shown. Monochromatic light passing through a single slit has a central maximum and many smaller and dimmer maxima on either side. In contrast, a diffraction grating produces evenly spaced lines that dim slowly on either side of center. Note that the central maximum is larger than those on either side, and that the intensity decreases rapidly on either side.
Figure 1 shows a single slit diffraction pattern. Light passing through a single slit forms a diffraction pattern somewhat different from those formed by double slits or diffraction gratings. Discuss the single slit diffraction pattern.
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