Doppler effect

Last updated 2023.02.26 Edit Source

• Physics
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• Supersonic
• Waves
• Sound

when a source is stationary, waves propagate from it with a uniform frequency in all directions
when a source is moving in a medium, the waves in the direction of its movement are generated closer together (the relative velocity of the wave and source is lower) → higher frequency
→ lower frequency behind the source

the velocity of the observer or source is relative to the medium
the total Doppler effect may be the result of the combinating the motion of the source, the observer, or the medium itself

discovered by Austrian physicist Christian Doppler in 1842

$$f=\left( \frac{c\pm v_{r}}{c\pm v_{s}} \right)f_{0}$$
$f=\text{observed frequency}$
$f_{0}=\text{emitted frequency}$
$c=\text{propagation speed of waves in medium }$
$v_{r}=\text{relative speed of reciever}$

added to c when $r\to s$, subtracted from c when $(s)r\to$,

$v_{s}=\text{relative speed of source}$

added to c when $(r)s\to$, subtracted from c when $s\to r$,

used to measure water current velocities (laser Doppler velocimeter (LDV) acoustic Doppler velocimeter (ADV))
can also be used for:

• air (SODAR)
• blood (echocardiogram)

aids drivers to determine the location and heading of emergency vehicles
if it is higher pitched → it is moving in the direction of the driver
if the pitch is lower → it is moving away
if pitch is changing → either relative speed is changing OR it is passing by ($v_{\text{radial}}=v_{s}\cos(\theta)$)

Doppler effect for EM waves → red- and blueshift
not the same as cosmological redshift ← doppler redshift will affect

used to detect close binary stars, measure rotational speed of galaxies, or to detect exoplanets using doppler spectroscopy

radar waves are sent and received to determine (relative) speed of object
$$\Delta f=\frac{2\Delta v}{c}f_{0}$$

DORIS

fast moving satellites → shift of frequency relative to ground station
frequency is changed dynamically to compensate

# Sonic boom

https://www.youtube.com/watch?v=kdiHmSWI2Ks
https://hubblesite.org/contents/media/images/4509-Image.html#:~:text=The%20universe%20is%20expanding%2C%20and,distance%20the%20light%20has%20traveled
https://www.nhs.uk/conditions/echocardiogram/#:~:text=An%20echocardiogram%2C%20or%20%22echo%22,different%20parts%20of%20the%20body
https://en.wikipedia.org/wiki/Doppler_effect
https://en.wikipedia.org/wiki/Doppler_spectroscopy
https://en.wikipedia.org/wiki/Sonic_boom
https://en.wikipedia.org/wiki/DORIS_(satellite_system)
https://en.wikipedia.org/wiki/Sodar