The lengthening of a transmitted signal's wavelength and/or a decrease in its frequency, which indicates that the object is moving away from the observer, is called the

That would be called the Doppler Effect.

The Doppler effect is the change in the frequency and/or wavelength relative to the movement of the source of the wave You would notice this effect when you hear sirens. As they move away there is a sudden change in pitch. This was named after the physicist who noticed this phenomenon, Christian Doppler. 

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