An observer will measure the wavelength of waves emanating from a source that is movingaway as longer than it really is.

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Quizplus · Accepted Answer

This is known as the Doppler effect, where the relative motion between the observer and the source of waves causes a shift in the perceived frequency/wavelength of the waves. If the source of waves is moving away from the observer, the waves will appear longer. This is known as the Doppler effect, where waves from a moving source appear to have a shifted wavelength and frequency to an observer. Away-moving sources have longer wavelengths, while toward-moving sources have shorter wavelengths.

An Observer Will Measure the Wavelength of Waves Emanating from a Source