A commuter train passes a passenger platform at a constant speed of 40.0 m/s. The train horn is sounded at its frequency of 320 Hz. What wavelength is detected by a person on the platform as the train approaches?
I'll assume 20 degC for which the speed of sound is
This is an example of the Doppler Effect.
For the person standing on the platform, more wavefronts are arriving per second so he or she will detect sound of a shorter wavelength and higher frequency.
If the source is approaching the observed wavelength is given by:
Putting in the numbers:
You can compare this to the note which someone would hear on the train:
So you can see that the person on the platform detects a slightly shorter wavelength and a slightly higher frequency of note.