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interference pattern (the cosine-squared factor), as shown in Figure 38.11. The Equation 37.2 indicates the conditions for interference maxima as d sin! " m%, where d is the distance between the two slits. Equation 38.1 specifies that the first (38.7) In Figure 38.11, d/a " 18 /m/3.0 /m " 6. Therefore, the sixth interference maximum
d " m d sin! " m
% % S E C T I O N 3 8 . 2 • Diffraction Patterns from Narrow Slits 1213 I Diffraction envelope Interference fringes –3 –2 – π π 2 3 /2 β π π π π Active Figure 38.11 The combined effects of two-slit and single-slit interference. This is the pattern produced when 650-nm light waves pass through two 3.0- / m slits that are 18 / m apart. Notice how the diffraction pattern acts as an “envelope” and controls the intensity of the regularly spaced interference maxima. Courtesy of Central Scientific Company At the Active Figures link at http://www.pse6.com, you can adjust the slit width, slit separation, and the wavelength of the light to see the effect on the interference pattern. Quick Quiz 38.3 Using Figure 38.11 as a starting point, make a sketch of the combined diffraction and interference pattern for 650-nm light waves striking two |