Example 18.5 Measuring the wavelength of light A double-slit interference pattern is observed on a screen 1.0 m behind two slits spaced 0.30 mm apart. From the center of one particular fringe to the center of the ninth bright fringe from this one is 1.6 cm. What is the wavelength of the light? PREPARE It is not always obvious which fringe is the central maximum. Slight imperfections in the slits can make the interference fringe pattern less than ideal. However, we do not need to identify the m = 0 fringe because we can make use of the fact, expressed in Equation 18.99, that the fringe spacing Ay is uniform. The interference pattern looks like the photograph of Figure 18.169. SOLVE The fringe spacing is 1.6 Ay = 96m = 1.78 x 103 m Using this fringe spacing in Equation 18.9IE', we nd that the wavelength is it: iAy %;\"\"(1.78x10-3m) 5.3 X 107 m = 530nm