Stage: 5 Challenge Level:
To see the shape of the wave fronts you might want to draw $5$ wave
fronts emitted by the aeroplane at times $t=0, 1, 2, 3, 4, 5$. The
orgin of each wave front will depend on the position of the
aeroplane when it is created; once created each will be
a circle whose radius grows at the speed of sound.