How Does Plane Polarized Light Oscillate Relative to Energy Transfer Direction?

[FelaKuti]

Homework Statement

In plane polarised light, the oscillations of the electric field are:

• 
  A. in a single plane, which includes the direction of energy transfer.
  B. in a single plane, which is perpendicular to the direction of energy transfer
  C. in perpendicular planes, which are perpendicular to the direction of energy transfer.
  D. in perpendicular planes, which include the direction of energy transfer

Homework Equations

[/B]
None.

The Attempt at a Solution

I believe I can justify ruling out C and D as plane polarised light would only have oscillations in one plane. My knowledge is that electric fields would always oscillate perpendicular to the direction of propogation, which would give me B. But I'm told it's A, can someone point me in the right direction to understand this?

 

[kuruman]

Draw a picture. Consider the plane containing E and the direction of energy transfer. Does it contradict what you already know?

 

[Javeria]

Can you please draw it? I don't understand this question at all
This is what the examiners report says:
Question 10: Answer B was most common. Students selecting this answer were thinking of the description of transverse waves after seeing reference to ‘perpendicular’. This would work if it said ‘and are’ instead of ‘which is’, but the direction reference is to the plane, not the oscillations.

 

[kuruman]

Two vectors define a plane. One vector is the electric field. What is the other vector? There is no mention of the B-field, but there is mention of the direction of energy transfer (Poynting vector). Answer A does not contradict the fact that the E-vector is perpendicular to the direction of propagation.

 

[Javeria]

Two vectors define a plane. One vector is the electric field. What is the other vector? There is no mention of the B-field, but there is mention of the direction of energy transfer (Poynting vector). Answer A does not contradict the fact that the E-vector is perpendicular to the direction of propagation.

Got it, thank youu

 

[haruspex]

I regard the term "plane polarisation" as misleading. Better is the alternative name, linear polarisation.
The oscillations of the electric field are along an axis orthogonal to the direction of travel, not really in a particular plane at all. It only looks like a plane when you plot that oscillation along the direction of travel.

 

[Jahnavi]

@kuruman , @haruspex , @ehild

I have two questions regarding polarisation .

1) Please help me understand the term "Plane of polarisation" .

The book first describes "Plane of oscillation" which is a plane in which vibrations of electric field are confined i.e a plane containing the electric field vectors and the direction of propagation of light .

Then the book says that a plane perpendicular to the plane of oscillation is called the plane of polarisation i.e a plane which does not contain vibrations of electric field vector .

What is the significance of plane of polarisation ? Isn't plane of oscillation sufficient to describe things ?

Does that mean that plane of polarisation is a plane which contains , magnetic field vectors of the polarised light and the direction of propagation ?

2) What happens to the magnetic field vectors after polarisation ? Since electric field vectors are confined in a plane, and magnetic field vectors are perpendicular to electric field , does that mean magnetic field vectors are also confined in a plane after polarisation ?

Thank you

 

[haruspex]

@kuruman , @haruspex , @ehild

I have two questions regarding polarisation .

1) Please help me understand the term "Plane of polarisation" .

The book first describes "Plane of oscillation" which is a plane in which vibrations of electric field are confined i.e a plane containing the electric field vectors and the direction of propagation of light .

Then the book says that a plane perpendicular to the plane of oscillation is called the plane of polarisation i.e a plane which does not contain vibrations of electric field vector .

What is the significance of plane of polarisation ? Isn't plane of oscillation sufficient to describe things ?

Does that mean that plane of polarisation is a plane which contains , magnetic field vectors of the polarised light and the direction of propagation ?

2) What happens to the magnetic field vectors after polarisation ? Since electric field vectors are confined in a plane, and magnetic field vectors are perpendicular to electric field , does that mean magnetic field vectors are also confined in a plane after polarisation ?

Thank you

Your book seems to be using an outdated definition of plane of polarisation. See https://en.m.wikipedia.org/wiki/Plane_of_polarization.

 

[Jahnavi]

I have already seen that link . I found difficult to understand that article .

Could you please answer my specific questions .

Thanks

 

[haruspex]

I found difficult to understand that article .

It is saying that when the term "plane of polarisation" was originally defined it was such that it contained the direction of propagation and the direction of the magnetic field vector. Thus, it was perpendicular to the electric field vector. But this was later considered misleading/inappropriate and other authors define it as containing the direction of propagation and the electric field vector. Your book seems to be using the older definition.

As I wrote in post #6, "plane of oscillation" is also misleading. The (electric) oscillation is along one axis. Consider a pendulum swinging east-west in a car traveling north. The oscillation is in the east-west axis. The path traversed by the bob lies in a horizontal plane, but there is no oscillation in the north-south axis.

Yes, if the axis of oscillation of the electric field does not rotate (is confined to a plane) then the same must be true for the magnetic field since the oscillation axes are always perpendicular to each other and to the direction of propagation.

 

[Jahnavi]

But this was later considered misleading/inappropriate and other authors define it as containing the direction of propagation and the electric field vector. Your book seems to be using the older definition.

As I wrote in post #6, "plane of oscillation" is also misleading. The (electric) oscillation is along one axis.

Even though it is misleading , but would you agree that with the newer definition of plane of polarisation , the plane of polarisation and plane of oscillation are same things ?

Yes, if the axis of oscillation of the electric field does not rotate (is confined to a plane) then the same must be true for the magnetic field since the oscillation axes are always perpendicular to each other and to the direction of propagation.

That means polarisation not only affects the electric field by restricting the electric field in one direction , it also restricts the magnetic field vectors in one direction (perpendicular to electric field vector ) ? ?

Quite strange that no book talks about magnetic field vector while discussing polarisation .

 

[haruspex]

with the newer definition of plane of polarisation , the plane of polarisation and plane of oscillation are same things ?

Yes.

polarisation not only affects the electric field by restricting the electric field in one direction , it also restricts the magnetic field vectors in one direction (perpendicular to electric field vector ) ? ?

Yes.

 

[ehild]

Homework Statement

In plane polarised light, the oscillations of the electric field are:

• 
  
  
  • 
    A. in a single plane, which includes the direction of energy transfer.
    B. in a single plane, which is perpendicular to the direction of energy transfer
    C. in perpendicular planes, which are perpendicular to the direction of energy transfer.
    D. in perpendicular planes, which include the direction of energy transfer

Homework Equations

[/B]
None.

The Attempt at a Solution

I believe I can justify ruling out C and D as plane polarised light would only have oscillations in one plane. My knowledge is that electric fields would always oscillate perpendicular to the direction of propogation, which would give me B. But I'm told it's A, can someone point me in the right direction to understand this?

You should consult the book about the definition of terms in the questions.
Otherwise @haruspex is right. We speak about linear polarization, when the electric field oscillates in one direction and it is said the direction of polarization, and it is perpendicular to the direction of propagation. For example, if the electric field is described by the formula $\vec E = E_x sin(ωt-kz) \hat x$ it is a wave polarized in the x direction and propagating in the positive z direction. But the field is the same all in a plane perpendicular to the z axis so it is a "plane wave"
Maxwell's equations give the relation between the electric and magnetic field vectors in free space. In case of a plane wave in the form $\vec E = \vec E_0 e^{i(ωt-\vec k\cdot\vec r)}$ , the Maxwell equation $\nabla \times \vec E = -∂\vec B/∂t$ means $\vec k\times \vec E = iω \vec B$ so the B vector is perpendicular both to the $\vec E$ vector and to the wave vector $\vec k$ which gives the direction of propagation. The same holds for the electric field according to the equation $\nabla \times \vec H = ∂\vec D/∂t$ (where $\vec D = ε\vec E$ and $\vec B = μ\vec H$).