The known expression for the Doppler frequency shift may be used in the derivation of the relation for interference-intensity patterns in Young's modified double-slit arrangement with a monochromatic source. Alternately, the statement that the interference-fringe maxima formed in the observation plane must move downward as a density gradient ahead of a collimating lens moves upward are sufficient to establish Doppler's law for the effect of velocity on frequency shift. The theoretical result has actually been confirmed by Schwar and Weinberg in experiments that were designed to demonstrate the use of schlieren interferometry with a laser source in velocity measurements on flames.