Doppler Effect Grade 12 Notes pdf (Physical Science Grade 12): Doppler Efect is the change in frequency (or pitch) of the sound detected by a listener because the sound source and the listener have different velocities relative to the medium of sound propagation.
Doppler Effect can also be the apparent change in frequency of a wave, as a result of relative motion between the source and the observer.
When the source of a sound is moving towards you, the pitch sounds higher than that of the source. When the source moves away from you the pitch sounds lower. This is known as the doppler effect.
Source moving towards a stationary listener
- Wavelength decreases/ waves are compressed.
- Frequency increases.
- Velocity constant.
- Pitch of sound increases.
Source moving away a stationary listener
- Wavelength increases/ waves are further apart
- Frequency decreases
- Velocity constant
- The pitch of sound decreases
Equation for Doppler Effect
Radar Guns: Doppler Effect
The traffic officers will point the radar gun to the oncoming traffic.
The radar gun will send the infrared waves to the oncoming car that is moving towards the traffic officer. The waves will be reflected back to the radar gun by the moving traffic. The radar gun then acts as the receiver of the waves. For classroom purposes, waves to be considered are the one coming from the traffic, so the oncoming traffic must be the moving source reflecting back waves to the radar gun. Radar gun becomes the receiver or listener. Now Doppler formular can be employed to solve for the unknowns
- Blood flow rate can be measured. (medical use)
- Speed of the planets and stars can be determined.
- Used to measure heartbeat of the unborn fetus in the womb. (medical use)
- Used in weather stations to detect precipitation.