Two coherent sources separated by a distance such that at a point their path difference is equal to one wavelength. What kind of interference occurs there?

Two coherent sources separated by a distance such that at a point their path difference is equal to one wavelength. What kind of interference occurs there?

• A. No interference
• B. Destructive interference
• C. Constructive interference
• D. Partial interference

Answer: (C)

When two coherent waves meet, the relative phase between them determines how their crests and troughs line up. If the path difference is an integer multiple of the wavelength, the waves arrive in phase—peaks line up with peaks and troughs with troughs. Their fields add constructively, so the amplitude is larger and the intensity is at a maximum. A path difference of one wavelength is exactly such a case, so this is constructive interference. (If the difference were half a wavelength, they would be out of phase and destructively interfere; the term partial interference isn’t the standard description in this context.)

When two coherent waves meet, the relative phase between them determines how their crests and troughs line up. If the path difference is an integer multiple of the wavelength, the waves arrive in phase—peaks line up with peaks and troughs with troughs. Their fields add constructively, so the amplitude is larger and the intensity is at a maximum. A path difference of one wavelength is exactly such a case, so this is constructive interference. (If the difference were half a wavelength, they would be out of phase and destructively interfere; the term partial interference isn’t the standard description in this context.)