This chapter is from the book
This chapter is from the book
This chapter is from the book
New Discoveries with Polarizers
Discover what polarized light and double refraction are.
Step By Step: Polarized Light
Supplies
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2 Polarizers (polarizing filters), found in photo shops (some sunglasses are also Polarizers)
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cover of a CD case or a piece of hard plastic
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laser pointer
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clear glass
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2 pieces of clear adhesive tape
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1 folded and crumpled sheet of cellophane
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1 ordinary flat mirror
Try looking through a polarizer at the light reflected by a CD case. (Hint: Pay special attention to the corners.) Rotate the CD case, then tilt it or look at it from a different angle. What happens? Now, look at the CD case in reflection, using an ordinary mirror. Do you notice anything special? Substitute a shiny piece of black plastic for the mirror. Compare the image formed by reflection and the object that is reflected. Is there anything weird in there? Turning the polarizer in front of your eye, look at the blue sky and a rainbow. (See Experiment 14, "Why Is the Sky Blue?") Look, too, in different positions at the reflections on a wall or window and find out why polarizers are used for cameras. Discover also why some fishermen use glasses with polarizing filters to better spot fish through the water. In a dark room, hold the polarizer against the laser end of a laser pointer pointed at a wall and see what happens to the laser light that hits the wall when you turn the polarizer. So many fascinating mysteries!
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A Step Further: Birefringence/Double Refraction
Look through both polarizers placed over a light source. Turn one of them until they both look dark (crossed polarizers). Place the plastic (CD case top) between the two polarizers, as the illustration shows, and see how the plastic is transformed. Tilt the plastic or rotate it and see what happens. Use a folded and crumpled piece of cellophane and repeat the previous experiment.
Stretch one piece of the adhesive tape before taping it down on the glass. Tape the other one down beside the first without stretching it. Now, place the glass between the two crossed polarizers and compare the two pieces.
Fun Facts: Polarized Waves
Light can be described in terms of transverse waves. You can produce transverse waves with a stretched rope, for example, by fixing one of its ends and moving the other end either up and down or sideways (linearly polarized waves), or moving it around in a circle, either clockwise or counterclockwise (circularly polarized light), as indicated in the figure. The oscillations you produce are perpendicular to the direction of propagation—hence the name transverse waves. Waves are characterized in general by a length (the wavelength), which measures how often a wave pattern repeats itself in space when you take a snapshot of the whole wave, and by a frequency, which measures how often the wave repeats itself in time when you consider a fixed point in space along the wave's trajectory.
Fun Facts: Polarizers
A polarizer works as a filter of oscillations. You can simulate a polarizer with two parallel broom handles close to each other. First, you produce a wave in a stretched rope (see figure). Then, you place the broom handles at a point where the transverse oscillation is greatest (an "antinode"). If you place them at a point where the cord is motionless (a node), the trick will not work! The "polarizer" lets pass through only oscillations along a certain direction (the polarizer axis) and eliminates the remaining oscillations. A circular polarized wave, for example, is transformed into a linearly polarized wave as it crosses a polarizer (see figure).
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Fun Facts: Light Waves
Light waves consist of oscillating transverse electric and magnetic fields. An electric field accelerates a charged particle and a magnetic field exerts a force on a moving charged particle when its velocity has a component perpendicular to the magnetic field. The light from the Sun and the light emitted by ordinary lamps is a mixture of waves with different wavelengths oscillating in all possible directions on a plane perpendicular to the direction of propagation (it is then unpolarized light). A real polarizer just lets the components parallel to its axis pass through and eliminates all other components. The polarizer thus produces linearly polarized light.
Fun Facts: Polarized Waves by Reflection and Refraction
When light strikes a surface, the reflection of light by the surface also produces linearly polarized light, especially at shallow incidence angles. The electrical charges on the surface act as tiny antennas. They respond differently to the oscillating electric field of an incoming light wave depending on its direction of oscillation. If the oscillation is parallel to the surface, the charges on the surface are pushed back and forth easily along the surface. In this case, the tiny antennas are very efficient and emit light linearly polarized along the horizontal direction. However, an electric field oscillating vertically pushes the charges up and down along the vertical direction. Since the charges can't leave the surface, their response is very poor at shallow angles of incidence. In transparent surfaces, as the incidence angle is equal or greater than a particular angle (the so-called Brewster angle), the emission of vertically polarized light is completely cut off. There is another way to see how light polarization is produced by reflection. At the Brewster angle, the electric field of the transmitted (refracted) light oscillates vertically along the propagation direction of the reflected wave. The tiny antennas cannot generate a reflected ray with vertical polarization, since the corresponding electric field oscillation would be along the direction of the wave propagation. Because the incident light has both polarizations, the reflected light becomes horizontally polarized. The same mechanism produces polarized light in rainbows (see figure showing a glass filled with water simulating a rainbow; a laser beam can be used to probe the polarization effect of the rainbow—be careful with the extra light rays coming out of the glass!). Experiment 14, "Why Is the Sky Blue," also demonstrates how partially polarized light is produced in a blue sky. Sunglasses and the polarizer of photographic cameras can thus dramatically reduce glare by absorbing most of the light reflected from a surface. (To accomplish that, what should the orientation of the polarizer's axis be?) Light reflected from a highway surface at Brewster's angle is often annoying to drivers and can be demonstrated quite easily by viewing the distant part of a highway, the reflections of the Sun's light from car windshields, or the surface of a swimming pool on a hot, sunny day. Modern lasers often take advantage of Brewster's angle to produce linearly polarized light from reflections at the mirrored surfaces positioned near the ends of the laser cavity. Now, considering that the Brewster angle varies with the frequency of the light components, can you understand what happens when you look at a CD case with a polarizer? Does a mirror produce polarized light? (Illuminate the mirror with a flashlight at different angles of incidence and check the reflected light with a polarizer.)
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Simulating the Polarization Effect in a Rainbow H
In a dark room, turn the polarizer (sheet of polaroid) to check whether the "rainbow ray" (laser beam coming out of the glass) is polarized. (It should be projected on a "screen," such as a white wall.) The dots represent the vertical polarization (perpendicular to the plane of the paper), and the double arrows stand for the horizontal polarization of the light's electric field.
CAUTION
To avoid accidents, place opaque obstacles at the spots where the reflected and the first refracted rays come out of the glass.
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Fun Facts: Birefringence (Double Refraction)
Birefringence occurs when a material displays two different refraction indexes for the horizontal and vertical polarizations of light. A light component with a polarization that affects the material most strongly has a higher refraction index and therefore travels slower than the light with the other polarization, associated with a lower refraction index. As a plastic is submitted to stress, it becomes birefringent. This can be demonstrated by using two crossed polarizers (sheets of polaroid). As unpolarized light crosses the first polarizer, it becomes linearly polarized along the polarizer's axis. Since the axes of both polarizers are perpendicular to each other, no light comes out from the second polarizer. If you place a piece of strained plastic in between the crossed polarizers, the polarized light coming out of the first polarizer is split into two components, one along the "fast lane" direction and the other along the "slow lane" direction. As the two wave components leave the plastic, there are several possibilities: the resulting wave coming out of the strained plastic can be linearly polarized, circularly polarized, and so on. (You can visualize the result of two perpendicular oscillations using a laser pointer, as suggested in Experiment 12, "Pictures of Sounds," under Playing with Sounds: Acoustics.) The several components of light are thus "filtered" by the second polarizer in different ways depending on how the plastic is strained and also by its thickness. Because the birefringence is related to the stress and is color-dependent, differently strained areas appear as different colors. If you bend a clear plastic ruler, does it display birefringence where it is bent? What happens when the ruler returns to its normal condition? Engineers use this trick to map out the strain distribution in plastic models of structures, such as bridges, submitted to variable loads. What about a CD case? When you glue pieces of rigid plastic or heat them up, do you produce birefringence?
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