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S E C T I O N 3 8 . 2 • Diffraction Patterns from Narrow Slits 1209 Quick Quiz 38.1 Suppose the slit width in Figure 38.6 is made half as wide. The central bright fringe (a) becomes wider (b) remains the same (c) becomes narrower. Quick Quiz 38.2 If a classroom door is open slightly, you can hear sounds coming from the hallway. Yet you cannot see what is happening in the hallway. Why is θ sin dark = 2 /a sin dark = /a sin dark = – /a sin dark = –2 /a L a 0 y 2 y 1 – y 1 – y 2 Viewing screen θ θ θ θ λ λ λ λ Figure 38.6 Intensity distribution for a Fraunhofer diffraction pattern from a single slit of width a. The positions of two minima on each side of the central maximum are labeled. (Drawing not to scale.) Example 38.1 Where Are the Dark Fringes? Light of wavelength 580 nm is incident on a slit having a Solution The problem statement cues us to conceptualize From the triangle in Figure 38.6, note that tan ! dark " y 1 /L. Because ! dark is very small, we can use the approximation sin ! dark ! tan ! dark ; thus, sin ! dark ! y 1 /L. Therefore, the positions of the first minima measured from the central axis The positive and negative signs correspond to the dark "y 1 " " 7.74 & 10 ' 3 m " To finalize this problem, 7.74 mm. # 3.87 & 10 ' 3 m " y
1 ! L sin !
dark " (2.00 m)(#1.933 & 10 ' 3 ) sin !
dark " # % " # 5.80 & 10 ' 7 m 0.300 & 10 ' 3 m " # 1.933 & 10 ' 3 note that this value is much greater than the width of the What If? What if the slit width is increased by an order of magnitude to 3.00 mm? What happens to the diffraction Answer Based on Equation 38.1, we expect that the angles dark for the m " # 1 dark fringes are The positions of the first minima measured from the central and the width of the central bright fringe is equal to 2 "y 1 " " 7.74 & 10 ' 4 m " 0.774 mm. Notice that this is smaller than the width of the slit. In general, for large values of a, the various maxima and minima are so closely spaced that only a large central bright " #3.87 & 10 ' 4 m y
1 ! L sin !
dark " (2.00 m)(#1.933 & 10 ' 4 ) sin !
dark " # % " # 5.80 & 10 ' 7 m 3.00 & 10 ' 3 m " # 1.933 & 10 ' 4 Investigate the single-slit diffraction pattern at the Interactive Worked Example link at http://www.pse6.com. Interactive |