When a ambulance drives past you at a high speed you can hear a change in the pitch of the sound. That change in the preserved frequency is called the Doppler effect. Whenever there is relative motion between a wave source and an observer, a Doppler shift in the frequency occurs.
The Doppler Effect is a straight forward concept to understand, let Alt Shift X explain and outline the basic idea.
Watch the pre-lecture videos and read through the OpenStax text before doing the pre-lecture homework or attending class.
Whenever a wave source and an observer of that wave are moving relative to each other, the observed frequency will be shifted from the original frequency. To visualize this effect you have to think about the wave fronts for a source that is moving.
In the above animation, imagine that is a wave from a police siren and the car is going to drive past you. In front of the car (before it's passed by) the wavecreast are crunched together. If you are to hold a pencil to the screen and verbally make a tick sound each time a wave crest passes by, you'll get a feel for the frequency observed in front of the police car. Do the same thing to a point behind the car and sound out the ticks as each wave creats passes by. It should be very obvious the rate, and thus frequency, is higher in front than behind. In the case of a siren as a car passes by, you hear the frequency go from a high pitch (higher than in the frame of the car) to lower pitch (lower than in the frame of the car).
The equation (in 1D) introduced here will handle cases of either a moving source or observer, or even both moving. It involves both the observed frequency $f_o$, the source frequency, $f_s$, and the speed of the wave relative to the medium v.
$f_o$ = $f_s$ $\left( \frac{ v \pm v_o }{ v \mp v_s } \right)$
$v_o$ = speed of observer relative to medium, choose (-) in the numerator if the observer is moving away from the source, (+) if the observer is moving towards the source
$v_s$ = speed of source relative to medium, choose (+) in the denominator if the source is moving away from the observer, (-) if the source is moving towards the observer
Sensemaking Protip: Use your conceptual understanding to determine if the frequency observed is greater than or less than the frequency of the source. Then make sure you pick the correct signs in the equation's numerator and denominator such that the fraction is less than or greater than 1.
Now, take a look at the pre-lecture reading and videos below.
