How does the amplitude on a EM wave graph correspond to the number of photons?

  • #1

Main Question or Discussion Point

Hello all! I am trying to understand ER on a more intuitive level. I can see the relationship between energy and frequency. The relationship between amplitude and photon number is less clear. So far I have E = hf. I understand that the intensity of light is a function of the number of photons. Can this be correlated to amplitude? If so, then how? Thank you.
 

Answers and Replies

  • #2
Drakkith
Staff Emeritus
Science Advisor
20,801
4,544
What amplitude are you referring to?
 
  • #3
5,601
39
  • #4
Physics Monkey
Science Advisor
Homework Helper
1,363
34
curiousatlarg,

I assume by amplitude you mean the magnitude of the electric field, let's call it E. In fact, E is related to the average number of photons via [itex] N \sim E^2[/itex]. Please note also the very important word average. A macroscopic electric field does not correspond to an exact number of photons but instead to something called a coherent state of photons (we can discuss this later if you want).

My claim above can be understood as follows. Let's think about a EM wave of fixed frequency. If you accept that the energy of one photon is [itex] h f [/itex] then the energy of N photons is [itex] N h f[/itex] and the rate of energy flow, which is proportional to the intensity, is [itex] I \sim c N h f[/itex] ([itex] c [/itex] is the speed of light i.e. how fast the photons are going). Now you also know that the intensity is given by [itex] I \sim E^2 [/itex] in a classical EM wave, so by equating these two forms you find that [itex] E^2 \sim N[/itex] up to lots of factors that you can work out or look up somewhere.

Hope this helps.
 
  • #5
Thanks. I was referring to both the electric and the magnetic fields. What you gave me so far is what I was looking for. I will work with it some.
 

Related Threads on How does the amplitude on a EM wave graph correspond to the number of photons?

  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
2
Views
330
Replies
1
Views
514
  • Last Post
Replies
3
Views
539
  • Last Post
Replies
9
Views
3K
  • Last Post
Replies
1
Views
2K
Replies
12
Views
867
Replies
4
Views
1K
Replies
1
Views
191
Top