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Calculating velocity of an approaching vehicle

Is it possible to design a device that calculates the velocity of an approaching vehicle if the speed of sound in air at that location  is known?


Asked Vibhava




Now the source is moving at the speed of sound...


Such a device is already in use. It is based on a phenomenon called Doppler Effect.


Get the details at the following Links.


  1. www.physicsclassroom.com/class/waves/u10l3d.cfm
  2. science.howstuffworks.com/science-vs-myth/everyday-myths/doppler-effect3.htm
  3. http://en.wikipedia.org/wiki/Doppler_effect



LCD & Doppler effect

Panchami asked:

  1. what is liquid crystal display
  2. What is Doppler Effect ? ? ?


LCD stands for Liquid Crystal Display. It works on the principles of polarization. By the selective alignment of liquid crystals light is blocked or allowed making the display transparent.

Wave travelling from one medium to another

Iqbal Lucky asked

“when a wave enters from air to water, its wavelength changes but

frequency remains unchanged. why”


Velocity of a wave = frequency x time period

V = ν λ

When a wave travels from one medium to another, its velocity changes and so also is its wavelength, but frequency; which is the number of vibrations produced per second is independent of the medium, remains constant.

However, their is a phenomenon called Doppler Effect where the apparent frequency (observed frequency) is different from actual frequency and is caused by the relative movement between the source and observer (listener)

Got a better answer?

Please post it as comment.

Doppler Effect

Hasan asks:

“What is Doppler Effect?”



Doppler Effect is the apparent change in frequency of a wave due to the relative motion between the source and the listener.

(Please come back soon for more details with figures and videos)

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