The Doppler effect is the change in frequency or wavelength of a wave relative to an observer who is moving relative to the wave source. It can occur with any kind of wave, including sound, light, or radio waves. This effect is a key principle used in technologies like radar, sonar, and even the study of distant galaxies!

When the source of the waves is moving, but the observer is stationary, the wavefronts appear to change as they reach the observer. Wavefronts are like the ripples you see when you throw a stone in the water; they move outward from the source. If the source is moving towards the observer, the wavefronts seem to sweep across the observer more quickly. This leads to an increase in observed frequency. If the source is moving away, the wavefronts appear more spread out, leading to a decrease in observed frequency.

Example: Sirens! 🚓

Imagine an ambulance moving towards you with its siren on. As it approaches, the sound of the siren seems to get higher-pitched. This is because the ambulance is compressing the sound waves, making the frequency higher. Once it passes you and starts moving away, the siren sounds lower-pitched because the sound waves are being stretched out, lowering the frequency.