# sound and electromagnetic wave

Discussion in 'Physics' started by sachin.tiger, Sep 30, 2010.

1. ### sachin.tiger Thread Starter New Member

Sep 24, 2010
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does the frequency of the sound wave(mechanical wave) and of an electromagnetic wave change during propogation?
further, can you help me understand what does amplitude of an em wave mean?

2. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
522
Yes, if the source and observer are moving relative to each other.

It is called the Doppler effect.

http://en.wikipedia.org/wiki/Doppler_effect

The amplitude is another word for the value of the peak value of the varying quantity - the electric or magnetic field in the case of an em wave.

3. ### sachin.tiger Thread Starter New Member

Sep 24, 2010
10
0
no, i mean, during propogation a wave loses some of its energy and by plank's thoery E=hv so does it conclude that as energy of each particle reduces while propogating it's frequency also reduces?
and one more ques does plank's thory apply equally well to mechanical waves?

4. ### logicman112 Active Member

Dec 27, 2008
69
3
for em and sound waves, the wave may interact with objects(with mass) which will change the frequency and energy.
For a mechanical wave, oscillating particles may have definite frequency and wavelength but we can not determine it or they may be random and statistical. Any way frequency is mentioned with an uncertainty and this centre frequency may change by the interaction.

5. ### JDT Well-Known Member

Feb 12, 2009
658
87
For sound waves, which are a mechanical vibration in a medium (usually air) the relative speed of the transmitter or receiver relative to the medium causes an apparent change in frequency - known as the Doppler shift.

For an electromagnetic wave such as light or radio, the velocity is always C relative to the transmitter and the receiver, so the effect is not Doppler but Time Dilation. (Although the effect is the same).

http://en.wikipedia.org/wiki/Time_d...nce_of_time_dilation_due_to_relative_velocity

So, when you get caught by that police radar trap, remember that it was because time was going at a different rate in your car compared to the police radar gun!

6. ### sachin.tiger Thread Starter New Member

Sep 24, 2010
10
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plz, there is no relative motion of the source and the reciever, does in the static situation (considering practicality) there occurs some loss in frequency?
And can anyone relate it to the plank's thoery that i have mentioned?

Nov 9, 2007
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8. ### DonQ Active Member

May 6, 2009
320
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Generally No.

Two main ways to reduce EM energy per area.
1) Disperse. As each wave radiates out from a source, each one covers more area. Same total energy, and same energy per photon, but less energy (number of photons) per area. No frequency change.

2) Absorption. As a wave moves, some of its energy is transferred to something else. In a particle perspective, a particle transfers its energy to something else. It's energy changes, maybe to zero, as particles around it increase their energy (energy neither created or destroyed, only a change in form). But all the other 'particles' maintain their original energy. No change in frequency, just fewer photons at the original frequency, less total energy of the original frequency. The reduction in energy is not distributed equally across the photons.

A place to see an extreme example of what you suggest is "scintillation" or "Gamma-camera", where the energy is converted from each high-energy photon to a large number of lower frequency photons.