Relation between intensity and amplitude

  • Thread starter albega
  • Start date
  • #1
75
0

Main Question or Discussion Point

When superposing waves in say double slit interference from two slits, I seem to have come across two approaches:
1. Sum the two waves in complex form to get the resultant amplitude, take the real part, and square to get the intensity, i.e I=[Re(A)]2
2. Sum the two waves in complex form to get the resultant complex amplitude. Then take the modulus squared to get the intensity, i.e I=AA*.
What is it that makes these two approaches give the same result. Mathematically they seem to be different to me. Also, where does the second come from? I know I=cε0E2 but this doesn't involve the modulus does it?
 

Answers and Replies

  • #2
RUber
Homework Helper
1,687
344
If you think of the complex A in the exponential form, ## A = r e^{i \theta} ##, then ## [Re(A)]^2 =( r \cos \theta) ^2##.
Also, ## AA^* = r e^{i \theta}r e^{-i \theta} =r^2##, therefore these are mathematically the same only when you are approaching head on (angle = 0 or pi).
 

Related Threads on Relation between intensity and amplitude

Replies
1
Views
27K
  • Last Post
Replies
7
Views
13K
  • Last Post
Replies
3
Views
778
Replies
1
Views
4K
Replies
1
Views
2K
Replies
1
Views
6K
Replies
6
Views
19K
  • Last Post
Replies
7
Views
12K
Top