A rope is held at the two ends so that vertical and horizontal polarized waves can be sent from one end to the other. The rope is then inserted between the two cookie coolers. When the two cookie coolers are aligned, they can pass a rope wave polarized in that direction but when they are crossed no wave can pass.

This can be used as an analogy to light waves, which are also transverse waves. Technically the mechanism of polarization in electromagnetic waves is somewhat different and more complex than this model. In an advanced class, this can be used in conjunction with Demonstration M7-01: MICROWAVES - POLARIZATION.

Two crossed polaroids, oriented vertically and horizontally, are placed in front of a goose-neck lamp, thereby preventing light from passing to the viewers. When a third polaroid is inserted between the two crossed polaroids at an angle of 45 degrees with respect to the original axes, light can be seen passing through the system.

This demonstrates that the electromagnetic field of which the light consists is a vector. The diagonal polaroid passes a component at 45 degrees with respect to the original light, and the second polaroid passes a component at 45 degrees with respect to the diagonal polaroid. The component of a component is actually perpendicular to the axis of the second original polaroid.

The real paradox involving this system involves an analysis of single photons. How can a single photon originally polarized parallel to the first polaroid have its angle of polarization rotated 90 degrees and exit the final polaroid polarized perpendicular to its original plane of polarization?

Compare M7-03, a simpler demonstration using only two polarizing filters.

Use a mirror to reflect the original dispersed light onto the screen, then move the support to the left to intercept the beam with the second prism, which will direct the beam onto the same spot after recombining. The optical element configuration is shown at the left (one prism) and at the right (two prisms) above The resulting "spectrum" for each configuration is shown below.

Three sources are permanently mounted on a roll-around cart, from top to bottom: (1)a clear glass long-filament incandescent light bulb which produces a continuous white light spectrum, (2) a mercury lamp which produces a line spectrum, and (3) a cadmium lamp which produces a line spectrum

These spectra are seen using 1"x2" sections of a large roll of replica diffraction grating material with 13,200 lines per inch. The pieces of grating material are relatively cheap, and may be given to the students. Tell your students to go away and look at the spectra of various lights.

The three lamps are mounted in a vertical line so the colors of the lines are the same as those in the adjacent white light spectrum. Point out that the spectra of mercury and cadmium are very different, and generalize that observation to suggest uniqueness of the spectra for each material.

This demonstration illustrates how real color films work.

Here's the question: What is greener than green? To find the answer, you must carry out the following experiment. Two slides are prepared, the first with the right half covered by a green and the left half by a magenta filter, the second with the right half covered by a green filter but with the left half open so it is white; both have a black dot in the center to stare at.

Everyone stares at the dot in the middle of the first slide for about 20 to 30 seconds. That slide is then quickly replaced by the second slide, with everyone continuing to stare at the black dot in the center. On the second slide the green half on the right is the same as the first slide, but the magenta left side of the first slide is replaced on the second slide by a white field. The white field viewed on the left side of the second slide is actually greener than the actual green field at the right.

Why do you see green on the right side of the second slide, and why is it greener than the actual green side? Staring at the first slide saturates your green receptors on the right side, but saturates your red and blue (magenta) receptors on the left side. When the second slide comes up, the green receptors on the right remain saturated (even more), causing the color to be washed out. However, the magenta side has saturated your red and blue receptors, but left your green receptors totally unused. Therefore, when the white light is seen, red and blue are washed out but the green receptors are strongly excited, leading to a beautiful green field.

To perform the experiment using your video monitor, click on the photograph above to get the first slide, then click on that slide when you have stared at it for 20-30 seconds to saturate your eyes. Prepare your computer to do the transitions rapidly by cycling through the photographs to place them in your cache memory or use the PowerPoint program linked below. However, this only works if your monitor is at the proper pixel size.