Doppler Effect — Definition
Definition
Imagine you're standing by a road, and an ambulance with its siren blaring approaches you, passes you, and then moves away. What do you notice about the sound of the siren? As it comes towards you, the pitch (how high or low the sound is) seems higher than usual.
As it passes you, the pitch suddenly drops, and as it moves away, the pitch seems lower than when it was stationary. This change in the apparent pitch or frequency of sound due to the relative motion between the source of the sound (the ambulance) and the listener (you) is what we call the Doppler Effect.
\n\nIt's not that the ambulance's siren is actually changing its frequency; it's emitting a constant frequency. The change you perceive is purely an 'apparent' change. This happens because when the source is moving towards you, the sound waves get 'compressed' in front of it.
This compression means the crests of the waves arrive at your ear more frequently, leading to a higher perceived frequency (higher pitch). Conversely, when the source moves away, the sound waves get 'stretched out' behind it.
The crests arrive less frequently, resulting in a lower perceived frequency (lower pitch). \n\nThe Doppler Effect isn't just for sound. It applies to all kinds of waves, including light. For light, instead of a change in pitch, we observe a change in color.
If a light source is moving towards us, its light appears 'bluer' (higher frequency, shorter wavelength), which is called a blueshift. If it's moving away, it appears 'redder' (lower frequency, longer wavelength), known as a redshift.
This principle is incredibly important in astronomy for determining if stars and galaxies are moving towards or away from Earth. \n\nCrucially, the Doppler Effect depends only on the *relative* velocity between the source and the observer, and the speed of the waves in the medium.
It doesn't matter if the source is moving, the observer is moving, or both are moving; what matters is their velocity relative to each other along the line connecting them.