# Amplitude of an electromagnetic wave

1. Oct 29, 2012

### jaydnul

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?

2. Oct 29, 2012

### antibrane

The intensity of an EM wave is written in terms of the amplitude:

$$I=cn \frac{\epsilon_0}{2}\left|E_0\right|^2$$

so these aren't really independent things.

3. Oct 29, 2012

### Baluncore

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

4. Oct 29, 2012

### Staff: Mentor

This is according to the quantum view of light.

It determines the intensity or brightness, in the classical view of light.

5. Oct 29, 2012

### sophiecentaur

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).