# Electromagnetic waves and phase difference

logearav

## Homework Statement

Electromagnetic waves contain time varying electric and magnetic field perpendicular to each other and also to path of progression. The phase difference between electric field vector and magnetic field vector is zero

## The Attempt at a Solution

How the phase difference is zero? Because when E is at ∏/2, B is at -∏/2. When phase difference is zero, Both E and B are in phase with each other, which means when E is at zero, B should also be at zero and when E is at ∏, B should also be at ∏. I am confused. Revered members, please help

## The Attempt at a Solution

Staff Emeritus
Homework Helper
Because when E is at ∏/2, B is at -∏/2.
Why do you say that?

logearav
Thanks for the reply. Now I have provided the image for EM wave and also another image --- two coloured curves representing two completely out of phase waves.
Both look similar. Then how can we say EM waves are in phase with each other?

#### Attachments

• EM.png
10.5 KB · Views: 663
• ART_active_force._eng.jpg
16.5 KB · Views: 505
Staff Emeritus
Homework Helper
The two illustrations, while looking similar, really have nothing to do with each other.

In the first one, you have two waves of different types which point in perpendicular directions. One is the electric field; the other is the magnetic field. They don't add to together or anything like that. Note that E and B peak at the same time, and they vanish at the same time. They oscillate in phase.

logearav
vela,
Now if i assume the coloured curves as a EM wave, Red representing E, and Blue representing B, can i say they are in phase?

Homework Helper
E and H are perpendicular. For example, E vibrates in the x direction and H vibrates in the y direction. You can not plot them in the same graph as the left picture in your attachment. There the curves have opposite phases, as one is minimum and the other maximum at the same time.
But E and H can not be compared this way. You can compare vectors by their magnitude, but can not say that the vector (2,0) is larger than (0,-2).

E and H in phase means that both have peak value at the same time and zero at the same time, as Vela pointed out.

ehild

logearav
@ehild,
I got it now. Thanks a lot.
@Vela
Thanks for clarifying. I read your post again with inputs from ehild and i got it now. Thanks a lot, Vela, for your reply.