<p align="right">Last Update: <font color="#4f81bd">January 02, 2025</font></p> ## BIG IDEAS - The Doppler effect is a change in the observed pitch of a sound or light, due to **relative motion** between the source and the observer. - Pointed out by [[Johann Christian Doppler]] in 1842. - Tested in 1845 by Christopher Heinrich Dietrich Buys-Ballot. - Determine velocities of solar prominences, double stars, rings of Saturn. - Used to determine distances of nearby stars. --- ### Textbook Alignment Walker 14-6 Doppler Effect Consider an ambulance, fire truck, or police vehicle with a siren blaring as it is moving toward you then passing you. - The [[Frequency|frequency]] is higher when the vehicle is moving toward you. - The [[Wavelength|wavelength]] is shorter (compression) - The frequency is lower as the vehicle moves away from you. - The wavelengths are low pressure (rarefaction). - This effect is known as the Doppler effect. ![[Doppler.png]] ### Moving Source For a stationary observer and a <font color="#f79646">moving sound source</font>, the perceived frequency ( $f_{obs}$ ) is $f_{obs} = f_s( \frac{v_w}{v_w \pm v_s}) \tag{1}$ Where: $f_s$ is the sound from a source, $v_s$ is the speed of the source, $v_w$ is the speed of sound. Use a minus sign for motion toward the observer and a plus sign for motion away from the observer. ### Stationary Source For a moving observer and a <font color="#f79646">stationary sound source</font>, the perceived frequency ( $f_{obs}$ ) is $f_{obs} = f_s (\frac{v_w \pm v_{obs}}{v_w})$ Where: $v_{obs}$ is the velocity of the observer. Use a plus sign for motion toward the source and a negative sign for motion away from the source. ### Slide Deck <div style="position: relative; width: 100%; height: 0; padding-top: 56.2500%; padding-bottom: 0; box-shadow: 0 2px 8px 0 rgba(63,69,81,0.16); margin-top: 1.6em; margin-bottom: 0.9em; overflow: hidden; border-radius: 8px; will-change: transform;"> <iframe loading="lazy" style="position: absolute; width: 100%; height: 100%; top: 0; left: 0; border: none; padding: 0;margin: 0;" src="https://www.canva.com/design/DAGPwHK-vYA/DLfNU3MIhygOssZeGLIrpA/view?embed" allowfullscreen="allowfullscreen" allow="fullscreen"> </iframe> </div> ### Related Topics --- [[Home|Home]] | [[Oscillations]] | [[Waves]] | [[Module 0 Sound]] | [[Module 1 Sound Waves]] | [[Module 2 Speed of Sound]] | [[Module 3 Sound Intensity]] | [[Module 4 Doppler Effect]] | [[Electromagnetic radiation]]