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Lab 5 - Properties of Light

# Lab 5 - Properties of Light - Physics 8B Lab 5 Properties...

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Physics 8B Lab 5 – Properties of Light rev 4.0 Lab 5 – Properties of Light Part I: Polarization Introduction Light is a transverse wave. It has the effect of creating an electric field and a magnetic field at various points in space. These fields are vectors and are perpendicular (transverse) to the direction of propagation of the light. By convention, we define the polarization of a light wave as the direction of its electric field. In light from most sources, the polarization (i.e. the electric field direction) changes randomly from moment to moment and from point to point. This kind of light is called unpolarized . In light from certain sources, however, the E-field oscillates back and forth along a fixed axis. This light is called linearly polarized . Since the E-field oscillates, we relax our definition some and usually talk about a polarization axis for the light (instead of direction). (Note there is also circular and elliptical polarization, but we won’t discuss that.) We will investigate polarization using a linear polarizer (a.k.a. polaroid), which is a piece of plastic where the long polymers have been stretched so that they all line up in one direction. When light of a certain polarization is incident on a linear polarizer, the component of the E-field in line with the polymer molecules is absorbed (i.e. it makes electrons move along the long polymers much like an antenna). The component of the E- field perpendicular to the molecules is not absorbed and thus transmitted . NOTE: We define the POLARIZATION AXIS of the polarizer as the polarization direction that is TRANSMITTED , (i.e. perpendicular to the polymers)

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Physics 8B Lab 5 – Properties of Light rev 4.0 Consider polarized light incident on a polarizer. Define θ , as the angle between the polarization of the incoming light and the polarization axis of the polarizer. If the incident light has E-field amplitude, E 0 , then after the linear polarizer, the light will be in line with the polarization axis of the polarizer and have E-field amplitude E 0 cos θ . Since intensity of a light wave is proportional to its electric field squared, if the incident intensity is I 0 , the intensity after the polarizer will be I 0 cos 2 θ . Consider unpolarized light incident on a polarizer. After the polarizer, the light will be in line with the polarization axis of the polarizer. But the intensity of light that is transmitted is different depending on all the possible values of θ . If you average over all possible values of θ , you find that the transmitted light after the polarizer has intensity I 0 /2. Activity 1: Single Polarizer On the optical bench, line up the optical bench light, a single polarizer and the white screen. 1.
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Lab 5 - Properties of Light - Physics 8B Lab 5 Properties...

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