Have you ever noticed that the noise of sirens from an emergency vehicle changes in pitch as it passes you? This is the Doppler Effect in action. Named after the Austrian mathematician Christian Johann Doppler who first proposed the idea of the phenomenon, the Doppler effect describes a change in frequency and wavelength of any waves, such as light or sound waves.

The Doppler Effect is used as a tool to study the motion or speed of objects such as the speed of a car on a highway, the motion of blood flowing through an artery, the rotation of a galaxy and even the expansion of the Universe. In fact, it was Edwin Hubble (who the big telescope is named after) who discovered that the universe expands as a consequence of the Doppler Effect.

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