Amplitude of an electromagnetic wave

AI Thread Summary
The discussion centers on the relationship between amplitude, intensity, and energy in electromagnetic (EM) waves, particularly visible light. It explains that the intensity or brightness of an EM wave is determined by the number of photons, while amplitude is linked to intensity in the classical view. In the quantum perspective, a photon represents energy carried by the wave, complicating the relationship between wave and particle models. The conversation highlights that amplitude is not independent and that the wave nature predominates, especially during interactions with materials. Ultimately, the mixed model of photons and waves presents challenges in understanding EM wave behavior.
jaydnul
Messages
558
Reaction score
15
Lets take visible light for example. The frequency/wavelength determines the amount of energy and the type of wave(micro,radio,gamma ect.) The intensity or brightness is determined by the amount of photons. So what does the amplitude determine?
 
Physics news on Phys.org
The intensity of an EM wave is written in terms of the amplitude:

<br /> I=cn \frac{\epsilon_0}{2}\left|E_0\right|^2<br />

so these aren't really independent things.
 
If you know it's energy then you are treating it as a single photon. Amplitude is meaningless.

If you know it's wavelength then you are treating it as a wave. As an EM wave it has both an E and an M component, each has it's own amplitude, the ratio of the E and M amplitudes is the impedance Zo of the material through which the EM wave is propagating. For free space Zo = 120 * Pi
 
lundyjb said:
The intensity or brightness is determined by the amount of photons.

This is according to the quantum view of light.

So what does the amplitude determine?

It determines the intensity or brightness, in the classical view of light.
 
If you want to consider photons at the same time as waves then you have a problem. A photon is not just a small point. It is just an amount of energy that the wave is carrying - or, at least, with which the wave interacts with objects. In pretty well every respect, it is the wave nature that dominates - except when there is an interaction involved. Any wave will not interact with a 'system' instantaneously. It takes time for the receiver (atom, molecule or TV set) to respond - several, or even many cycles of the wave are involved (depending on the 'Q' of the system) so how can this relate to a model involving a 'shower' of little photons, each one with its own 'phase'? This is a mixed model and it is neither fish nor fowl but I understand that it is attractive at a stage in the learning of the way EM works, despite being not very fruitful (imho).
 
This is from Griffiths' Electrodynamics, 3rd edition, page 352. I am trying to calculate the divergence of the Maxwell stress tensor. The tensor is given as ##T_{ij} =\epsilon_0 (E_iE_j-\frac 1 2 \delta_{ij} E^2)+\frac 1 {\mu_0}(B_iB_j-\frac 1 2 \delta_{ij} B^2)##. To make things easier, I just want to focus on the part with the electrical field, i.e. I want to find the divergence of ##E_{ij}=E_iE_j-\frac 1 2 \delta_{ij}E^2##. In matrix form, this tensor should look like this...
Thread 'Applying the Gauss (1835) formula for force between 2 parallel DC currents'
Please can anyone either:- (1) point me to a derivation of the perpendicular force (Fy) between two very long parallel wires carrying steady currents utilising the formula of Gauss for the force F along the line r between 2 charges? Or alternatively (2) point out where I have gone wrong in my method? I am having problems with calculating the direction and magnitude of the force as expected from modern (Biot-Savart-Maxwell-Lorentz) formula. Here is my method and results so far:- This...
Back
Top