The Doppler Effect

I have always loved this idea in physics. Like how a sound changes as it goes by. The Doppler effect (or Doppler shift), named after an Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave (for example, a sound wave) for an observer moving relative to the source of the waves. It is commonly heard when a vehicle with a siren approaches, passes and recedes from an observer. The received frequency is increased (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is decreased during the recession. The vehicle is pushing up against the medium in front (air) as it goes making it "thicker" and creating a lag behind causing the waves to elongate.