what's the meaning of amplitude in EM wave?

[particle]

i learnt that EM wave is varying electric field and magnetic field .But what's the meaning of the amplitude of EM wave? The displacement of the photon or the magnitude of the electric/magnetic field(no. of photons)?

[robphy]

Classically, the square of the amplitude of a wave is proportional to the energy [density].

[particle]

but it doesn't have its own meaning?
in water wave, the amplitude is the magnitude of the variation of water molecular.
then what's varying in the EM wave?

[ZapperZ]

but it doesn't have its own meaning?
in water wave, the amplitude is the magnitude of the variation of water molecular.
then what's varying in the EM wave?

In the simplest form, it's the amplitude of the E-field vector. This is what typically defines the polarization of an EM wave.

Zz.