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The Doppler's Effect

Have you wondered why you hear a high pitch sound from an ambulance siren as it approaches you, and lowers as it moves away? This occurs because of the Doppler Effect.

Terms to know:

  1. Sound waves: The movement of energy that travels through different mediums like air and water.

  2. Frequency: Number of wave cycles that occur per unit of time.

  3. Wavelength: Distance between two consecutive points of a wave.

What is it ?

The Doppler Effect is a phenomenon that describes the change in the frequency of a sound wave, depending on if the observer is moving.

In the case of sound waves, this effect causes the pitch of the sound to be:

  • Higher when the sound source is moving towards the observer.

  • Lower when the source is moving away from the observer.

This occurs because the waves are either:

  • Compressed together (shorter wavelength and higher frequency), giving a higher pitch.

  • Stretched apart (longer wavelength and lower frequency), giving a lower pitch.

Doppler Effect can also be a change in the frequency of a light wave:

  • Higher pitch signifies the colours blue and violet, due to their shorter wavelengths and higher frequency.

  • Lower pitch signifies the colours red and orange due to their longer wavelength and lower frequency.

How can it be used ?

1. Speed Cameras - to measure the speed of moving vehicles. This is done by bouncing a radar (type of wave) off a moving vehicle, the camera can detect the speed from the reflected waves by analysing them to see how much of their frequency is shifted.

2. Astronomy - to determine whether an object is moving towards or away from Earth. This is done by analysing the frequency shift of the object.

3. Medical Ultrasound - to measure blood flow and detect any abnormalities in the body. This is done by measuring the change in frequency of the sound waves reflected off the blood cells. Doctors are then able to diagnose diseases in the patient.

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