Light Polarization (page 2)

Author: Janice VanCleave

Try New Approaches

How is light affected by nonpolarized lenses? Repeat the experiment using the lenses from an inexpensive pair of nonpolarized sunglasses.

Design Your Own Experiments

    1. Unpolarized light becomes partially polarized when reflected from nonmetallic surfaces, such as water or glass, while some is transmitted and/or absorbed. The parallel components of the incident light (the light striking a surface) is largely reflected. The reflected light is partially polarized in a direction parallel with the surface it reflects from. Thus for lakes and other surfaces parallel to Earth's surface, the reflected light is said to be horizontally polarized. There is a special angle of incidence (the angle between the incident light and a line perpendicular to the surface it strikes) called polarization angle at which the reflected light from a nonmetallic surface is completely polarized. Design a way to determine the approximate magnitude of the polarization angle for light reflecting from water. One way is to fill a large bowl with water and set it on a table. In a darkened room, hold a flashlight above the water so that its light is perpendicular (normal or 90°) to the water's surface, then slightly tilt the flashlight. Sit facing the bowl and view the water's surface through a polarized lens. Since the water acts as a polarizer, the lens acts as an analyzer. Part of the light will enter the water and form a light spot on the bottom of the bowl, and part will reflect, thus forming an image (a representation of a physical object formed by light reflected from a surface) of the end of the flashlight. Rotate the analyzer to determine if the reflected light from the water's surface is polarized. The degree of polarization will be determined by how much of the reflected light (the image) disappears. If you see no change in the image, it indicates that the reflected light is not polarized. If all of the image disappears, then the reflected light is 100% polarized. Increase the angle of incidence by tilting the flashlight from its perpendicular position and analyze the reflection at the new angle. Continue this until the flashlight is held parallel to the water's surface. Estimate the angle of the flashlight from perpendicular (the incidence angle) and devise a way to compare the degree of polarization for each angle, such as a rating of 0 to +5, with +5 being the greatest polarization. For information about polarized angles see Craig F. Bohren's What Light through Yonder Window Breaks? (New York: Wiley, 1991), pp. 37–40. For information about horizontal and vertical polarization see Louis A. Bloomfield's How Things Work (New York: Wiley, 1997), pp. 494, 498, and 529–530.
    2. How does the material of the reflective surface affect polarization? Repeat the previous investigation using different materials including a smooth metal, such as a flat baking pan. For information about polarization from reflected surfaces, see of Karl F. Kuhn's Basic Physics (New York: Wiley, 1996), pp. 79–80.
    1. A material is said to be optically active when it rotates the plane of the electric light waves passing through it. Design a way to test the optical activity of materials, such as by placing a material between two polarized lenses. For example, to test the optical activity of a plastic cup, place the lamp, cup, and analyzer in a straight line with one another. Rotate the analyzer and observe the changing patterns of color. To test the optical activity of transparent tape, you can stretch some of the tape across a small, open frame cut from poster board. Repeat the experiment replacing the cup with the frame of tape. For information about the colors seen in optically active materials when viewed through an analyzer see Hazel Rossotti's Colour: Why the World Isn't Grey (Princeton, N.J.: Princeton University Press, 1993), pp. 49–51.
    2. Sugar and tartaric acid (cream of tartar) are known to be optically active. Design a method of testing different concentrations of solutions of water and these chemicals. Note that the container used to hold the solutions must not be optically active. Test light passing through empty containers for optical activity and select one that is not optically active. Devise a method of comparing the degree of optical activity of each concentration.

Get the Facts

When two polarized lenses are placed together, the amount of light passing through the analyzer is greatest when their crystals are parallel and least when they are perpendicular, such that no light gets through. Since the light exiting the polarizer is polarized, why does any of it pass through the lenses when the crystals in one lens are positioned between 0° and 90° relative to the crystals in the second lens? What are photons and how does their spin affect polarization? For information see John Gribbin's In Search of Schrödinger's Cat (New York: Bantam, 1984), pp. 218–229.

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