C.3.8—Two-source interference

Set up two-source interference

Two coherent sources emit waves with the same frequency and a constant phase difference. At each observation point, compare the two source-to-point distances to find the path difference.

Map bright and dark regions

For in-phase sources, path difference $n\lambda$ gives constructive interference and a bright or high-amplitude region. Path difference $(n+\tfrac12)\lambda$ gives destructive interference and a dark or low-amplitude region.

Read the pattern

Points equidistant from the two sources have zero path difference and form a central constructive line. Further maxima and minima occur where the path difference changes by half-wavelength steps.

Common trap

Do not use source-to-source separation as the path difference. It is the difference between the two travel distances to the same observation point.