# Chapter 29 - Think and Explain - Page 560-561: 33

By an odd number of half-wavelengths: $\frac{1}{2}$ wavelength, $\frac{3}{2}$ wavelength, $\frac{5}{2}$ wavelength, etc.

#### Work Step by Step

This is discussed on pages 549-550, and illustrated in Figure 29.12. If one wave is delayed by a half-wavelength, the two waves will arrive out of phase, and cancel each other. Delaying that wave by a whole number of wavelengths on top of the half-wavelength delay doesn't change anything; the two waves will still be out of phase.

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