Light Polarization

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Description of the Experiment

This experiment allows you to measure the cross-polarization result of light. Two polarizers with variable angles are used in series over a white LED and the light intensity is measured.

Light can be described as an electromagnetic wave with a property called polarization due to the electric field oscillation in the plane orthogonal to the propagation direction. When the oscillation is just over a single direction, the light is said to be polarized. Certain materials have the property to block the wave except along this precise direction. The objective of this experiment is to reveal how this occurs regarding the light intensity.

Links

  • Video: rtsp://elabmc.ist.utl.pt:554/polaroide.sdp
  • Laboratory: Intermediate e-lab.ist.eu
  • Control Room: Polaroide
  • Grade: **


Experimental Apparatus

The apparatus consists on a light source (high bright white LED) passing a collimator, which focuses the light rays into a parallel beam of light. At the beginning of the optical path, a vertical light polariser can be interposed.

In the optical path, light travels through two polarized lenses without graduation, having the angle of one of them been pre-set and being the other one free to rotate around the axis of propagation.

The light is finally collected through a converging lens into a photo-diode that measures the incident radiation intensity. This intensity is obviously the result of attenuation introduced by polarizing systems brought into its optical path.


Protocol

In this control room we can measure the attenuation of a light beam caused by the cross-rotation of two polarised lenses. This beam can be selected from the light source or can be previously polarized.

The supervisor of the experiment can choose two sweep limits for one polarizer and set the angle of the second polarizer acquiring the value of the transmitted power in a photo-diode.

The resolution (angle increment between two samples) can be chosen according to the interest of the control room supervisor.

Advanced protocol

The experience allows to be performed with starting with polarized light. Selecting this option the user can check the Malus's law in which multiple polarisers are used. In such case we need to multiply all the squares of the cosines between themselves, so the final value of the attenuation equation became:

[math] I_s = I_a \sum cos ^ 2 (\alpha_i) [/math]

were \( \alpha_i \) are the successive polarisers angles and \(I_a\) the initial light intensity.

Links