Light Polarization
Contents
Description of the Experiment
This experiment allows you to polarize light from an incoherent source (a white LED) using a polarizer and study the effect caused by a second polarizer on the same beam of light, ultimately measuring the incident power on a photocell.
Light can be described as an electromagnetic wave with a characteristic polarization, where its electric field oscillates in a specific plane perpendicular to the direction of its propagation. Certain media have the property of absorbing the wave in one direction on that plane and remaining "transparent" in the other direction, such as "Polaroid" lenses.
The aim of this experiment is to demonstrate the effect of light passing through various polarizers by interposing them in the light optical path at various angles defined by the user. However, when there are a chain of three polarizers, we the interpretation of the effect of the polarizers can be quantum, especially when dealing with a single photon.
Links
- Laboratory: Intermediate elab.tecnico.ulisboa.pt
- 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 \prod cos ^ 2 (\alpha_i) [/math]
were \( \alpha_i \) are the successive polarisers angles and \(I_a\) the initial light intensity.
