Background

An antenna receives an induced current from the electromagnetic field of the passing wave. The dipole is a linearly polarized antenna, sensitive to signals oriented in a particular direction. In this experiment, we can see this dramatically, as changing the orientation of the antenna relative to the source produces a significant drop in signal strength, so that it is no longer receiving sufficient energy to light the bulb.

Compare this effect to other wave and polarization demonstrations in sections G3 and M7.

The microwave transmitter and receiver are positioned about 50 cm apart, with both antennas oriented vertically, and the signal is seen using an overhead projector microammeter. Rotate the receiving antenna to demonstrate that these antennas have a direction of polarization associated with them. Q: When the wire cookie cooler grate is placed between the source and the receiver with its wires oriented vertically, what will happen to the microwaves? Will they (a) be attenuated, or (b) pass on through unaffected? What happens when the grate is placed in the beam with its wires horizontal? A: The beam will be attenuated when the wires are vertical and pass through unaffected when the wires are horizontal!

This perhaps counterintuitive result is just the opposite from what one would expect on the basis of the demonstration with rope waves, M7-05: ROPE AND COOKIE COOLERS. Here, the microwaves are attenuated because when the wires are vertical the vertically polarized electric field is absorbed, causing electron currents in the vertical wires. The radiation is re-emitted, but in all directions, so the intensity is attenuated due to solid angle. The iodide crystals with which light polaroid is constructed also absorb the light this way, but the electron currents produced are absorbed in the crystal rather than being re-emitted.

Cross the transmitting and receiving antennas so that no radiation is picked up. Demonstrate the component of a component by holding the wire grate between the transmitter and the receiver at a 45 degree angle.

For a polarized light beam which is then analyzed by rotating a second polaroid in front of the first, the intensity is proportional to the square of the cosine of the angle between the polarizer and the analyzer. This is known as Malus' law.

The laser beam is polarized by the first polaroid, and the intensity is measured using a radiometer as the analyzing polaroid is rotated from 0 degrees to 90 degrees. Actually, the laser beam is partially polarized, so the first polaroid is just completing the job.

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.

A single ray of light enters a semicircular plastic slab and is reflected and refracted by the internal flat surface. Brewster's angle is the angle at which the internally reflected ray is completely polarized in the plane perpendicular to the optical board surface (parallel to the internal surface of the plexiglass slab off which the internal reflection occurs). This occurs when the angle between the refracted and the internally reflected rays is exactly 90 degrees (photograph at left). This can be verified by rotating a polaroid sheet in the light ray coming from the source at the left. When the polarization axis of the polaroid sheet is vertical no internally reflected light is visible (center photograph). Note that the refracted ray is partially polarized at this angle, as seen when the polarizing sheet is rotated so that its axis is vertical (photograph at right).

The lens is used to keep the light ray to a minimum size. Use of the semicircle assures that the incoming and reflected rays will enter and exit normal to the surface and experience no bending.

This gizzit contains a series of glass plates mounted at the Brewster angle, about 57 degrees. Unpolarized light entering one end confronts a series of surfaces, each of which reflects a fraction of the incident light which is polarized perpendicular to plane of the incident and reflected rays. After several such reflections the light exiting the other end is polarized in the plane of the incident and reflected rays (well, almost polarized).

A bright light can be directed through this device onto a screen for group viewing.

The gizmo in the picture contains a collimated light source and a reflecting dielectric surface at approximately the Brewster angle, which can be rotated as a unit about a vertical axis, and a second reflecting dielectric surface. The light emerges to strike a screen, as shown in the sequence of photographs below. The light reflected off the lower reflector is polarized in the plane of the reflector, which can be verified by rotating the light/reflector unit so that the two reflecting surfaces bend the beam in orthogonal planes or by inserting a polarizing sheet between the two reflectors.

Conducting (metallic) surfaces can be substituted for the dielectric surfaces, showing that there is no polarization in reflection from conducting surfaces.

The sequence of photographs above show the following physical situations:

-two dielectric surfaces with parallel surfaces.

-two dielectric surfaces with perpendicular surfaces.

-two conducting surfaces with parallel surfaces.

-two conducting surfaces with perpendicular surfaces.

-lower conducting surface parallel to upper dielectric surface.

-lower conducting surface perpendicular to upper dielectric surface.

When sunlight reflects off a horizontal dielectric surface such as water in a lake, wet roads, or even dry smooth roads, the reflected light is largely horizontally polarized. Polaroid sunglasses are oriented vertically so they remove "glare," which is horizontally polarized specular reflection from such surfaces.

Position the point source so that it reflects from the lecture table onto the front white screen at about the Brewster angle. Rotating a polaroid in the light from the point source before reflection shows clearly that the reflected light is polarized. Individually viewing the reflected light directly using a polarizing filter demonstrates the value of polaroid sunglasses in removing glare. Placing a piece of aluminum foil where the light hits the table demonstrates that reflection from a conducting surface is not polarized.

The photographs above show the polarization axis (a) parallel and (b) perpendicular to a dielectric surface, and (c) parallel and (d) perpendicular to a conducting surface (a sheet of aluminum foil).