The Doppler effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source.

It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

In this paragraph, what is 'relative'?

The position of the observer compared to the source.

Explanation

According to this passage, the Doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. This indicates a difference in the movement and therefore the position of the observer and source.

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