Kevin is standing outside a railway station. he can know whether a train is approaching the station or departing from the sound it makes. which principle of sound waves is he using? resonance beats doppler effect interference

If Kevin is good, really good, he can use the principle of the
Doppler effect to distinguish the sound of an approaching train
from the sound of a departing train.

-- The sound of a train APPROACHING the station will have a HIGHER
pitch than the sound actually emitted by the train.

-- The sound of a train moving AWAY from the station will have a LOWER
pitch than the sound actually emitted by the train.

But the skill that Kevin will need is to exactly know the sound actually
emitted by the train ... without hearing it ... and to be able to compare
that to the sound that he DOES hear at the moment.  Not easy. 

The independent variable in this experiment is the angle at which the light from the ray box strikes the mirror.hope this .