Why is the intensity of polarised light equal to half of the incident light?

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Discussion Overview

The discussion centers on the phenomenon of polarized light and the relationship between the intensity of polarized light and the intensity of incident light. Participants explore the reasoning behind the assertion that the intensity of polarized light is half that of the incident light, examining both theoretical and conceptual aspects of polarization.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the explanation that half of the incident light is parallel and half is perpendicular to the plane of polarization, suggesting that light is incident at various angles and that only a small fraction would be parallel.
  • Another participant introduces the concept that the intensity of transmitted light varies as cos²(θ), noting that the average value of cos²(θ) over all angles leads to the conclusion that half of the intensity is transmitted.
  • A participant seeks clarification on the origin of the cos² function, particularly in relation to the electric field direction of the light.
  • A further explanation is provided regarding the electric field components of light, indicating that the component parallel to the polarizer passes through while the perpendicular component is blocked, emphasizing that intensity is proportional to the square of the electric field.
  • One participant suggests that electromagnetic radiation can be split into components, and by symmetry, these components should be equal, reiterating that the intensity relates to the angle of the polarizer and the square of the electric field.

Areas of Agreement / Disagreement

Participants express differing views on the explanation of why polarized light has half the intensity of incident light. There is no consensus reached, as some participants challenge the initial reasoning while others provide alternative explanations involving mathematical relationships.

Contextual Notes

The discussion includes assumptions about the distribution of light angles and the mathematical treatment of intensity in relation to electric field components. The implications of these assumptions on the overall understanding of polarization are not fully resolved.

Who May Find This Useful

This discussion may be of interest to those studying optics, electromagnetic theory, or anyone seeking to understand the principles of light polarization and intensity relationships.

nolanp2
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the intensity of polarised light is equal to half of the intensity of the light before it has been polarised, and this has been explained to me as being a result of half of the incident light being parallel and half perpendicular to the plane of polarisation, and so half passes through and half is blocked.

This does not make sense to me, as i would have thought that most of the light would be incident at all angles between 0 and pi, and only a very small amount would happen to be parallel to the P of P.

Can somebody help me out with a better explanation please?
 
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The passed intensity varies like cos^2. The simple half and half argument works because the average value of cos^2 is 1/2.
 
i see. so can you explain exactly where the cos squared functions come from, possibly in terms of the electric field direction of the light?
 
Suppose the light travels in the z direction. Then the electric field [itex]\vec E[/itex] is perpendicular to the z-direction, at some angle [itex]\theta[/itex] with respect to the x-direction.

Now suppose the polarizer axis is along the x-direction. Then the component of the electric field along the x-direction ([itex]E \cos {\theta}[/itex]) makes it through, but the component along the y-direction ([itex]E \sin {\theta}[/itex]) is blocked.

Finally, in general, the intensity of the light is proportional to the square of the electric field in the wave.
 
You can split the EM radiation of light into x and z components. by symetry these components should be equal. as jtbell said, the amount blocked by the polarizer on each axes is relative to the angle of the polarizer and the intensity is the square of the radiation.
 

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