Vibrations in One Direction
Unpolarized light consists of waves with electric fields vibrating in all directions. In some cases, however, all the waves in a beam vibrate in one direction or in one plane. Such light is said to be polarized.
In this project, you will investigate polarized and unpolarized light. You will determine what effect an analyzer has on polarized light. You will determine if the angle of incidence (angle at which light strikes a surface) affects the degree of horizontal polarization. You will also test the optical activity of different solid materials and of different concentrations of water solutions.
Purpose: To polarize light.
- desk lamp with incandescent bulb
- inexpensive plastic polarized sunglasses
- Turn on the lamp and position it so that the bulb is visible to you. Stand at a distance of about 3 feet (1 m) from the bulb. Look at the bulb and make note of its brightness.
- Remove the lenses from the sunglasses by twisting the frames and popping the plastic lenses out.
- At a distance of about 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.
- Hold the second lens (called lens B) in front of but not touching lens A and while still closing one eye look at the light through both lenses. Hold lens A in place while rotating lens B until the bulb appears at its brightest as viewed through both lenses. Then slowly rotate lens B 90°, observing any change in the brightness of the bulb.
The bulb is less bright when viewed through one lens than with the naked eye. 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 only partly visible.
Visible light is a 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 the propagation of the wave). A transverse waveis 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 directions that are not parallel with one another, such as the light from the bulb in this experiment. (See Figure 55.2.) A polarized lens acts as a polarizer, which is a material that allows electric fields of light vibrating in only one direction to pass through it. When unpolarized light strikes a polarized lens, part of the light is reflected (bounced off of), part is absorbed by the lens, and part of the light with electric fields vibrating in one specific plane passes through. The light that emerges from the other side of the polarizer contains light waves with electric fields vibrating only in one direction or in one plane and is said to be polarized (also called plane polarized). Figure 55.2 represents vertical polarization. Polarized sunglasses are generally made of plastic material in which needlelike microscopic crystals are embedded. These crystals line up parallel to one another and make the polarized lens act as though it consists of many slit-like openings parallel to one another. So only those light waves with electric fields vibrating in the same plane as the parallel slits in the polarizer can get through.
Placing the two lenses together demonstrates the effect of using two polarizers aligned with each other. 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 90° 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, as shown in Figure 55.3. Science Fair Hint: Use diagrams such as the ones shown to represent the results of this investigation.