What is the apparent change in frequency of waves due to the motion of a source? photoelectric effect prismatic change doppler effect wavelength variability

Doppler effect

Explanation:

The Doppler effect refers to the change in a wave perceived frequency when the emitter of the waves, and the receiver (or observer in the case of light) move relative to each other.  

In other words, it is the variation of the frequency of a wave due to the relative movement of the source of the wave with respect to its receiver.  

Another important aspect is that the effect occurs in all waves (including light and sound). However, it is more noticeable to humans with sound waves.

The Doppler effect is the apparent change in frequency of waves due to the motion of a source.

the answer is 12 as i may remember

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3. dark matter

empty space could also be a simple vacuum