# About electromagnetic waves

1. Nov 15, 2009

### mak_phy

I dont really know how to put my doubt in words but I will try.

Electromagnetis waves of various frequencies are used for

communication.Also there are EM waves from other sources, for

example even sunlight is eloctromagnetic wave.Now all these

waves are out there in space. So at any one point in space, it

will be kind of a resultant of all EM waves at that point.

[Like may be if you mix water from various sources ]

So what I think is if it were possible to see the electromagntic

field at any point, we would end up seeing one resultant value.

Now how does receiving antenna seperate the wave it wants?
I was syudying science at one time, but forgot, so please

clear my doubt.And bear with me if my question doesnt make sense.

2. Nov 15, 2009

Welcome mak phy,
Rather than "see the electromagnetic field at any point" I would prefer to use the expression detect the field at that point and what you detect depends upon the nature of the detector used.
I am a bit out of my sphere of knowledge with antenae but as far as I know the induced current in a receiving antenna has a maximum value for those frequencies for which there is a resonance effect,that is from transmitter antenae where there is a length and frequency match.The antenna also responds to other frequencies but less strongly and it is the tuning part of the circuit that separates the frequencies.
Hopefully others will come in with better informed comments.When I get some spare moments I will read up on it.

3. Nov 15, 2009

### Staff: Mentor

Hi mak_phy, welcome to PF,

Dadface is right, any detection or measurement apparatus is sensitive to a particular range of frequencies. Take for example your eyes. They are sensitive to EM waves in the visible range, but not for example in the radio frequency range. So even though a hypothetical measuring device which was sensitive to both ranges would be able to detect the total field as being the sum of the two (called "superposition") a radio receiver will not detect the visible light waves and your eyes will not detect the radio waves.

4. Nov 15, 2009

### mak_phy

Thank you Dadface and DaleSpam for your replies.
I think I got what you are explaining theoretically. Both the resonance part and eyes responding only to certain frequencies. It seems even after interference, individual frequencies dont lose their existence/identity.
I got confused because of the fact that a square wave being made of different sinusoids - all we see is a square wave.

5. Nov 15, 2009

### mikelepore

It's the same with sound. The pressure exerted by the air on someone's eardrum has just one value at any one point in time, and a sequence of values in a duration of time. Out of that pattern we can identify a low-pitched tuba, a high-pitched flute, human voices, bird chirps, car horns, footsteps, heard all at the same time, and we hear the separate E, C, and G notes of a C-major chord.

6. Nov 15, 2009

### mak_phy

Thank you both of you for the "LIGHT" and "SOUND" analogy .
That explains it all.