How to Control the Transmission of Polarized Light?

The Idea

The orientation of the crests and troughs of light waves can be horizontal, vertical, or anything in between. Unpolarized light consists of a random mix of orientations. Polarized light has only one direction. This gives it the unique properties used in liquid crystal displays found in many television screens and computer monitors. Sunglasses reduce glare by allowing only selected orientations of polarized light through. This experiment explores how to identify whether light is polarized and how the transmission of polarized light can be controlled.

What You Need

• 2 polarized sheets
• calculator or other LCD display, such as a laptop computer
• polarized sunglasses
• light source
• shallow tray
• water
• few rocks
• sheet of glass
• optional: protractor, light sensor

Method

Transmission through polarized sheets

1. Take one of the polarized sheets. Hold it in front of a light source (a lamp or an open window) and rotate it. Turn the sheet a full 360 degrees, holding the sheet so it remains roughly perpendicular to your field of view.
2. Try this with the other sheet.
3. Now, with both sheets, hold them in front of the light, one in front of the other, and rotate only one of them. Observe what happens.Add in other combinations: rotate both in the same direction, rotate both, but in different directions. Describe the effect of the sheets. Is the light from your light source polarized?
4. Using a nondestructive method, such as applying a small piece of tape, identify an edge on each, which when placed together,blocks the maximum amount of light.
5. Note: This is good to do using an overhead projector or the light from an LCD projector.

Reflections

1. Place a small flat mirror face up on a table.
2. Place a light source on one side of the mirror.
3. View the reflected light through one of the polarized sheets as you rotate the sheet. Is that light polarized? View through a range of reflected angles from nearly perpendicular to a very glancing angle to the mirror.
4. Place the two sheets, one on top of the other, but with taped edges aligned, so both sheets have the same polarization plane. What happens when you rotate the sheets, both with respect to the mirror and to each other?
5. Repeat 1–4, using light reflected from a square of glass.

Laptop/sunglasses

1. Hold a polarized sheet in front of the LCD (liquid crystal display) of a laptop computer or digital calculator.
2. Rotate the polarized sheet. What can you conclude about the LCD of the laptop?
3. Take the two lenses from an old (no longer needed) pair of polarized sunglasses. Hold them—one in front of the other—and view a light source. Are the glasses polarized? You also can try this with two pairs of polarized sunglasses.

Expected Results

Light will pass through polarized sheets with little loss when the directions of polarization for both sheets line up. As the sheets are rotated, more and more of the light is blocked. With the direction of polarization of the two sheets at right angles, almost no light can pass through, as shown in Figure 86-1. This can be quantified in Malus's law, which is addressed later in this section.

A light meter is a good way to quantify the amount of light passing through a filter. Figure 86-2 provides an approximate visual reference to evaluate the amount of light transmission through a set of polarizing filters.

Reflected light can be polarized. This can be determined by observing the effect of a single polarized filter on reflected light.

The light from a LCD, such as a laptop screen, is also polarized. This can be seen by rotating a polarized filter, such as polarized sunglasses, in front of an LCD screen, as shown in Figure 86-3 and Figure 86-4.

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