27.8 Polarization  (Page 6/19)

Glass and plastic become optically active when stressed; the greater the stress, the greater the effect. Optical stress analysis on complicated shapes can be performed by making plastic models of them and observing them through crossed filters, as seen in [link] . It is apparent that the effect depends on wavelength as well as stress. The wavelength dependence is sometimes also used for artistic purposes.

Another interesting phenomenon associated with polarized light is the ability of some crystals to split an unpolarized beam of light into two. Such crystals are said to be birefringent    (see [link] ). Each of the separated rays has a specific polarization. One behaves normally and is called the ordinary ray, whereas the other does not obey Snell’s law and is called the extraordinary ray. Birefringent crystals can be used to produce polarized beams from unpolarized light. Some birefringent materials preferentially absorb one of the polarizations. These materials are called dichroic and can produce polarization by this preferential absorption. This is fundamentally how polarizing filters and other polarizers work. The interested reader is invited to further pursue the numerous properties of materials related to polarization.

Test prep for ap courses

In an experiment, light is passed through two polarizing filters. The image below shows the first filter and axis of polarization.

The intensity of the resulting light (after the first filter) is recorded as I . Three configurations (at different angles) are set up for the second filter, and the intensity of light is recorded for each configuration. The results are shown in the table below:

Set up	Angle of second filter compared to first filter	Intensity of light after second filter
Configuration A	θ 1	I
Configuration B	θ 2	0.5 I
Configuration C	θ 3	0

Complete the table by calculating θ ₁ , θ ₂ , and θ ₃ .

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Section summary

• Polarization is the attribute that wave oscillations have a definite direction relative to the direction of propagation of the wave.
• EM waves are transverse waves that may be polarized.
• The direction of polarization is defined to be the direction parallel to the electric field of the EM wave.
• Unpolarized light is composed of many rays having random polarization directions.
• Light can be polarized by passing it through a polarizing filter or other polarizing material. The intensity $I$ of polarized light after passing through a polarizing filter is $I=I_0{\text{cos}}^2\mathrm{\theta ,}$ where $I_0$ is the original intensity and $\theta$ is the angle between the direction of polarization and the axis of the filter.
• Polarization is also produced by reflection.
• Brewster’s law states that reflected light will be completely polarized at the angle of reflection ${\theta }_{\text{b}}$ , known as Brewster’s angle, given by a statement known as Brewster’s law: $\text{tan}{\theta }_{\text{b}}=\frac{n_2}{n_1}$ , where $n_1$ is the medium in which the incident and reflected light travel and $n_2$ is the index of refraction of the medium that forms the interface that reflects the light.
• Polarization can also be produced by scattering.
• There are a number of types of optically active substances that rotate the direction of polarization of light passing through them.