Purpose
To polarize light.
| 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. |
Remove the lenses from the sunglasses by twisting the frames and popping the plastic lenses out. |
| 3. |
At a distance of 3 feet (1 m), close one
eye and look through one of the polarized lenses at the lit bulb and again note the bulb's brightness.
This will be called lens A. |
| 4. |
Hold the second lens (called lens B) in front of but not touching lens A and look at the light through both lenses. Hold lens A in place while rotating lens B until the bulb appears at its brightest when viewed through both lenses. Then slowly rotate lens B 90 degrees 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 in front of the other lens, the light decreases still further until
it is no longer visible or is only partly visible.
Why?
Visible light is form of radiation, which is energy that travels in the form of electromagnetic waves (transverse waves consisting of an electric field and a magnetic field vibrating at right angles to each other and to the direction of propagation of the wave.) A transverse wave is one in which the vibrations are perpendicular to the direction in which the waves are traveling, like the up-and- down, or vertical motion of water waves. But unlike water waves, light waves can vibrate in all directions perpendicular to the direction of motion.
Polarized light is light in which the electric fields of the light waves vibrate in a direction parallel to each other. A light wave whose electric field is vibrating in the vertical direction is said to be vertically polarized. A light wave whose electric field is vibrating in the horizontal direction is said to be horizontally polarized.
Unpolarized light contains light waves with electric fields vibrating in different directions, such as the light from the bulb in this experiments. A polarized lens acts as a polarizer, which is a material that allows electric fields of light vibrating in only one direction or one plane to pass through it. When unpolarized light strikes a polarized lens, part of the light is reflected, part is absorbed by the lens and part with electric fields vibrating in one specific plane passes through. The light that emerges from the other side of the polarizer as polarized light. Polarized sunglasses are generally made of plastic material in which needlelike crystals are embedded. These crystals line up parallel to one another and make a polarized lens act as though it consists of many slitlike openings parallel to one another, so only those light waves with electric fields vibrating in the same plane as the parallel slits get through.
Placing the two polarized lenses together demonstrates the effect of using two polarizers aligned with one another. The first lens in line with the light is called the polarizer, and the second lens is called the analyzer (a polarizer used to determine if light is polarized). When the crystals in the two lenses are lined up parallel to one another, the greatest amount of light possible passes through. In this position, rotating the analyzer (lens B) 90 degrees results in the crystals in the separate lenses being at right angles to one another. None of the polarized light is able to pass through the analyzer in this position.
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