# plane of polarization

Discussion in 'Physics & Math' started by perfectionist, Apr 6, 2007.

1. ### perfectionistRegistered Senior Member

Messages:
32
1)I have a basic doubt on polarization. It is given in book that the plane of polarization is perpendicular to the plane of vibration. What was need for defining the plane of polarization to be perpendicular to the plane of vibration? When I searched the internet, I found a site which says that the plane of polarization is the plane in which the magnetic field vector is oscillating and plane of vibration is the plane in which the electric field vector oscillates. In an electromagnetic wave, the electric field vector is perpendicular to the magnetic field vector. So this sounds to be true, but I would like have the opinion of this forum.

When I referred Griffith's book on Electromagnetism I came across the fact that the polarisation vector defines the plane of vibration. Doesn't this mean that the plane of vibration is parallel to the plane of polarization? It also says that an electromagnetic wave propagating across the z-direction with its electric field vector along the x-direction and magnetic field vector along the y direction is said to be polarized in the x-direction.(Because by convention we use the direction of Electric field vector to specify the polarization of an electromagnetic wave).

3. ### James RJust this guy, you know?Staff Member

Messages:
31,829
I'm, not sure what you mean by "plane of vibration".

The polarisation of a light wave is defined with reference to the electric field vector. So, for a plane-polarised wave propagating in the z direction, with the electric field in the x-z plane and the magnetic field in the y-z plane, the wave is polarised in the x direction.

5. ### leopoldValued Senior Member

Messages:
17,455
yes, it's true

example one:
if electricity is flowing left to right then the magnetic field will expand outward. the lines of force will flow from the +'s to the .'s, thus
. . .
>>>> , electron flow
+++

the > represents electron flow.

example 2 is the opposite of example 1, thus:
+++
<<< , electron flow
. . .

does that clear up anything for you?

Last edited: Apr 7, 2007

Messages:
10,296
Hello, Perfectionist,

Perhaps I can help you a little here, I spend a number of years working with microwaves and waveguides.

Although it IS an arbitrary assignment, it was decided long ago to use the plane of the E (electromagnetic) wave component as determining the polarization of the EM radiation. I'm sure you're aware that the E and H (magnetic) fields exist at 90-degrees to each other and both move forward together anong the y-axis of transmission.

Does that help or do you have furthur questions?

8. ### perfectionistRegistered Senior Member

Messages:
32
That piece of information was really helpful. Now, how do we define the principal section of a crystal--- A plane containing the optic axis and the poynting vector.Is there a better and simple way of defining it? Can it be defined as the plane containing the optic axis and the plane of incidence?

Last edited: Apr 7, 2007
9. ### Gavin R. PutlandRegistered Member

Messages:
1
Historical background: HUYGENS defined a PRINCIPAL SECTION of a calcite crystal as a plane perpendicular to a natural surface and parallel to the axis of the obtuse solid angle (the optic axis). MALUS, who discovered polarization by reflection, defined the PLANE OF POLARIZATION of a polarized ray as the plane, containing the ray, in which a principal section of a calcite crystal must lie in order to cause only ordinary refraction. This plane, as we now know, happens to contain the MAGNETIC vectors, not the electric vectors. FRESNEL, in order to explain Malus's discovery in terms of transverse waves, found it necessary to suppose that the vibrations of polarized light were normal to Malus's "plane of polarization". So began the convention that the "plane of vibration" is normal to the "plane of polarization". Fresnel's "vibrations", as we now know, are vibrations of the ELECTRIC vectors, which have more interaction with matter than the magnetic vectors have. That tends to reinforce the convention that the vibrations are normal to Malus's "plane of polarization".

But if you didn't know about Malus's definition, you would assume that the "plane of polarization" contained the electric vibrations. Thus two clashing conventions have arisen.

More details and twists: What century is this? — the ambiguity of ‘plane of polarization’.