How are frequency and wavelength defined for electromagnetic waves?

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Frequency & wavelength of EM wave!

Hi..on some other thread i found this link showing animation of propagation EM wave:

http://www.molphys.leidenuniv.nl/monos/smo/index.html?basics/light_anim.htm

it has cleared my one doubt, because i used to think that EM wave is like an invisible line in air which moves up and down like sinusoidal manner. But it actually a variation in electromagnetic field.

But then my question is:
the frequency of the EM wave is the frequency of electric & magnetic field variation( as AC voltage)?
and how wavelength is measured since there is no(physical) peaks to measure distance between?

 

[Born2bwire]

The electromagnetic field is a force field. We can directly observe it by measuring the resulting force acting on a test charge. How this is done can vary. For example, an easy method is to use an antenna probe. In such a way we can easily probe the electromagnetic field in both time and space to find the peaks and thus we can easily see that the wavelength and frequency are both physical properties of an electromagnetic wave. The wavelength determines the spatial periodicity of the wave, like the peak to peak distance. The frequency determines the temporal periodicity of the wave, like the time between a peak to peak observation.