The Doppler effect is a change in the frequency of a
wave (sound, light, or other waves) that takes place whenever relative motion
exists between the source and a detector. This effect takes its name from the
Austrian physicist Christian Johann Doppler, who first stated the physical
principal in 1842.
A familiar example of this phenomenon, is the change in
pitch that can be heard by an observer listening to the whistle of an
approaching (or receding) train. In fact, in this case, Doppler's principle
explains why, if the source of sound (emitting a constant pitch) is moving
toward an observer, the sound seems higher in pitch, whereas if the source is
moving away it seems lower.
Christian Doppler applied enunciated his principle in relation to star movements. This principle is extensively used in many medical devices including fetal heart rate monitors and other cardiac ultrasound devices. Click on the hyperlink above to get an idea about the Doppler effect. Press on the P icon and next on the I icon.Move the human figure towards the plane and observe the pitch change, Also increase the speed of the plane and listen to the sonic boom. Note when the plane approaches the human the sound waves are compressed in the leading edge and widened at the trailing edge.