How to show light from laser pointer is plane polarised?

In summary, the answer key is the light is directed through one polarising filter then filter is rotated and the light changes intensity.
  • #1
songoku
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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
 
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  • #2
songoku said:
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?
 
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  • #3
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.
 
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  • #4
Steve4Physics said:
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.

Steve4Physics said:
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
 
  • #5
songoku said:
a) the maximum will be I and the minimum will be zero.
Yes.

songoku said:
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.
 
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  • #6
Thank you very much for the help and explanation Steve4Physics and kuruman
 
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1. How do you determine if light from a laser pointer is plane polarised?

To determine if light from a laser pointer is plane polarised, you can use a polarising filter and rotate it in front of the laser beam. If the intensity of the light does not change, then the light is plane polarised.

2. What is the difference between linearly polarised and plane polarised light?

Linearly polarised light refers to light that has its electric field oscillating in a single direction, while plane polarised light refers to light that has its electric field oscillating in a single plane. In other words, all plane polarised light is linearly polarised, but not all linearly polarised light is plane polarised.

3. Can light from a laser pointer be unpolarised?

No, light from a laser pointer is always polarised to some degree. This is because the laser light is emitted from a single source, which creates a coherent beam with a specific polarisation direction.

4. What factors affect the degree of polarisation in light from a laser pointer?

The degree of polarisation in light from a laser pointer can be affected by the angle of the polarising filter, the quality of the laser, and any obstructions or reflections in the path of the laser beam.

5. How is polarisation of light from a laser pointer useful in scientific research?

Polarisation of light from a laser pointer is useful in various scientific research fields, such as optical microscopy, spectroscopy, and telecommunications. It can also be used to study the properties of materials, such as their refractive index and birefringence.

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