(a) A representation of a shock wave produced when a source moves
from S0 to Sn with a speed vS , which is greater than the wave speed v in the medium. The envelope of the wave fronts forms a cone whose apex half-angle is given by sin 0 # v/vS . (b) A stroboscopic photograph of a bullet moving at supersonic speed through the hot air above a candle. Note the shock wave in the vicinity of the bullet.