# Polarization of em wave

1. Nov 17, 2009

### astro2cosmos

i read this "in electrodynamics, polarization characterizes em waves, such as light, by specifying the direction of the wave's electric field" in a book.
i really didn't get it clear & does the magnetic filed nothing to do with polarization?

2. Nov 17, 2009

### Born2bwire

The electric and magnetic fields are always normal to eachother for an electromagnetic wave. In a source-free homogeneous region of space, the electromagnetic waves can only be transverse waves. This means that the electric and magnetic field vectors are both normal to the direction of propagation. So, if we specify the vector direction of the electric field and propagation, then we have implicitly specified the direction of the magnetic field. By convention, we generally choose the electric field as the direction of reference for polarization.

Electric and magnetic fields are both vector fields. Not only do they have a magnitude and phase associated with each point in space, but they also have a direction. This direction is related to the direction that a test charge would travel from the force induced by the fields. For example, the Lorentz force on a test charge due to the electric field is simply
$$\mathbf{F} = q\mathbf{E}$$
So that is why there is a direction associated with the fields (the relationship with the magnetic field is a bit more complicated, the cross product of the particle's velocity and magnetic field).

So the polarization of the electromagnetic wave is a way of describing the orientation of the electric field vector component of the wave as the wave propagates through space and time.

3. Nov 17, 2009

### astro2cosmos

how the polarization will affect by change in the direction of electric field?

4. Nov 17, 2009

### Redbelly98

Staff Emeritus
What do you mean? The polarization is the direction of the electric field.

5. Nov 17, 2009

### Andy Resnick

The magnitude of the electric field is roughly 'c' times larger than the magnitude of the magnetic field; that is why the electric field is used.

6. Nov 17, 2009

### Staff: Mentor

In an EM wave the E and B fields are always at right angles to each other, so when you specify the direction of one, it automatically determines the direction of the other. We could use either one as the "polarization direction", but by convention we use the E field, to be consistent.

7. Nov 18, 2009

### astro2cosmos

is there any polarization is generated by magnetic field ??? likewise electric field.
& why polarization is only cause of eletric field?

8. Nov 18, 2009

### Redbelly98

Staff Emeritus
Yes, if the wave is polarized then both the magnetic and electric fields are polarized.

As jtbell said, it is by convention. By widespread agreement, the electric field is used to specify the polarization direction. If you are told what that direction is, then you know the direction of both the electric and magnetic fields.