Unit 1: Doppler Effect

IMPORTANT FACTS CONCERNING DOPPLER EFFECT

 

Doppler Effect Equation

  • The Doppler effect holds not only for sound waves but also for electromagnetic waves, including microwaves, radio
    waves, and visible light.
  • Police use the Doppler effect with microwaves to determine the speed of a car, a radar unit emits microwaves of certain frequency f toward the oncoming car. The microwaves reflected from the metal portions of the car back to the radar unit have a higher frequency f ‘ due to the motion of the car relative to the radar unit. The radar unit translates the difference between f’and f into the speed of the car, which is then displayed for the operator to see.
  • Ultrasonic waves (ultrasound) are sound waves with a frequency greater than 20 000 Hz (the upper limit of human hearing). These waves can be used in medicine to determine the direction of blood flow. The device, called a Doppler flow meter, sends out sound waves. The sound wave scan travel through skin and tissue and will be reflected by moving objects in the body (like blood). The reflected waves return to the flow meter where its frequency (received frequency) is compared to the transmitted frequency. Because of the Doppler effect, blood that is moving towards the flow meter will change the sound to a higher frequency and blood that is moving away from the flow meter will cause a lower frequency.

  • Ultrasound can be used to determine whether blood is flowing in the right direction in the circulation system of unborn
    babies, or identify areas in the body where blood flow is restricted due to narrow veins. The use of ultrasound
    equipment in medicine is called sonography or ultrasonography.

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