How to show light from laser pointer is plane polarised?

[songoku]

Homework Statement

A teacher has a laser pointer. The laser pointer emits a narrow beam of plane-polarised light. Describe how the teacher can demonstrate that the light from the laser pointer is plane polarised.

Relevant Equations

Malus Law

The answer key is the light is directed through one polarising filter then filter is rotated and the light changes intensity.

I don't understand how that proves that the light is plane polarised. I think if the light is unpolarised, the intensity will also change when it passes through polarising filter so how can we tell the difference between unpolarised and plane polarised light?

Also, by "plane polarised", does it mean fully polarised or partially polarised or can be both?

Thanks

 

[Steve4Physics]

The answer key is the light is directed through one polarising filter then filter is rotated and the light changes intensity.

I don't understand how that proves that the light is plane polarised. I think if the light is unpolarised, the intensity will also change when it passes through polarising filter

If unpolarised, the transmitted intensity won't change as the angle changes. The transmitted intensity will remain constant – it will be half of the incident intensity, whatever the angle is.

Suppose the incident intensity is I and you change the angle. What can you say about the maximum and minimum transmitted intensities when:
a) the incident light is fully plane polarised?
b) the incident light is partially plane polarised?

 

[kuruman]

Although I have not tried it, I suppose one could bounce the beam off a dielectric surface at Brewster's angle, rotate the laser and watch the reflected intensity. No analyzer needed.

 

[songoku]

If unpolarised, the transmitted intensity won't change as the angle changes. The transmitted intensity will remain constant – it will be half of the incident intensity, whatever the angle is.

Ah ok, now I understand. I misinterpreted the answer.

Suppose the incident intensity is I and you change the angle. What can you say about the maximum and minimum transmitted intensities when:
a) the incident light is fully plane polarised?
b) the incident light is partially plane polarised?

a) the maximum will be I and the minimum will be zero

b) sorry I don't know for this one

Thanks

 

[Steve4Physics]

a) the maximum will be I and the minimum will be zero.

Yes.

b) sorry I don't know for this one

Sorry, I should have been clearer. It depends on the 'mix'. Suppose the intensities of the polarised and unpolarised incident light are U and P respectively. The total incident intensity is I = U+P.

Half the unpolarised always gets through. Between 0 and P of the polarised light gets through. The min. and max. transmitted intesities are therefore U/2 and P + U/2.

 

[songoku]

Thank you very much for the help and explanation Steve4Physics and kuruman