Physics for kids 13+

Polarization: Vibrations in One Direction
Purpose To polarize light.

Materials
desk lamp with incandescent bulb
2 pairs of polarized sunglasses

Procedure
1. Turn on the lamp and position it so that the bulb is visible. Stand at a distance of about 3 feet (1 m) from the bulb. Look at the bulb and make note of its brightness.

2. Put on one pair of the sunglasses. 

 
3. At a distance of 3 feet (1 m), close one eye and look through one of the polarized lenses at the burning bulb and again note the bulb's brightness. This will be called lens A.

4. Hold the second pair of glasses so that one lens (called lens B) is in front of but not touching lens A. Look at the light through both lenses. Rotate lens B until the bulbs appears at its brightest when viewed through both lenses. Then, slowly rotate lens B 900 observing any change in the brightness of the bulb.

Results
The bulb is less bright when viewed through one lens. Viewing the bulb through two lenses further decreases its brightness. As one of the lenses is rotated over the other lens the light decreases until it is no longer visible or is only partly visible. 

Why?
Polarization of light refers to the direction of the electric field in an electromagnetic wave of light. A wave whose electric field is oscillating in the vertical direction is said to be vertically polarized. A wave whose electric field is oscillating in the horizontal direction is said to be horizontally polarized. The electric field in light waves from the sun vibrates in all directions, so direct sunlight is unpolarized.  When unpolarized light passes through a polarized lens, called a polarizer (material that polarizes light), all the light waves are absorbed by the lens except those vibrating in one plane. The light that emerges from the polarizer contains light vibrating only in one direction or one plane and is said to be polarized also called plane polarized.  Polarized sunglasses are generally made of plastic material in which needle-like crystals are embedded. These crystals line up parallel to each other and make a polarized lens act as though it consists of many slit-like openings  parallel to each other. So only those light waves vibrating in the same plane as the parallel slits in the polarizer get through.

Placing the two polarized lenses together demonstrates the effect of using two polarizers in line with one another. The first lens in line with the light is called the polarizer and the second lens is called the analyzer (a polarized material used to determine if light is polarized). When the crystals in the two lenses are parallel with each other, the most light possible passes through. In this position, rotating the analyzer (lens B) 900  results in the crystals being at right angles to one another. None of the polarized light is able to pass through the analyzer in this position, as shown in the figure.