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.

Compare J3-23: Faraday Cage - Radio Waves, which can use the same radio to illustrate a related phenomenon.

Invite students to place their hand through the image, to get a feel (or not) for what's happening. Have them consider where such images might be used.

Invite different students to view the tank from different angles and draw what the rod looks like. Have them compare their experiences and discuss.

A laser beam is spread into a horizontal line by a cylindrical lens, and passes over a current-carrying wire aligned along the original laser beam. The hot wire causes the air to have a very strong decreasing temperature gradient a long the central section of the laser light line, so that section bends upward. The bent laser line is displayed on the wall or screen across a long distance (perhaps the width of the room), as seen below.

Upward bending of blue light from the sky as it propagates along sun-heated sand in the desert causes blue light to appear to be coming from the ground, creating the classic illusion of a lake in the desert.

A video camera is optionally available to make this more visible in large lecture halls.

A clear plastic tank with a plugged spout is elevated above a second, shallower tank. The upper tank is filled with water. A laser is aligned so that it passes through the upper tank and is centered on the spout.

When the spout is unplugged, the water streams out into the lower tank. Several internal reflections of the laser beam should be visible in the outgoing stream of water, down to the point where it becomes too turbulent to see clearly.

As the water level in the tank drops, the water flow becomes so slow that the stream bends too sharply and there is no internal reflection, and the laser beam ceases to follow the flow of water.

Background:

The laser here is illustrating internal reflection: depending on the index of refraction of the water and the angle the light hits it at, more light can be reflected back and forth within the stream of water than passes through it. At a certain point, it exhibits total internal reflection, where essentially all of the light is traveling along the stream rather than heading straight out the side.

This demonstration can beneficially be used in combination with demonstrations of fibreoptic technology such as L5-13 or L5-23.

One end of the fiber optics strand is held pointed into a laser beam, and the other end held by a student who is free to move about. The light emerging out the other end can easily be seen over the entire lecture hall, even when some of the cable is coiled up.

Sections of fiber optics cable are used which were left over from the installation of fiber optics cabling in underground conduits all over the University of Maryland campus. These cables are now being used for both data and video transmission. The fiber optics strands below have adapters so that they can be plugged into an optical source directly.

A slide contains an array of 1, 2, 3, 4, and 5 slits. The single and double slit patterns are familiar. For N slits, where N is 3 or more, N-2 maxima of lesser intensity appear between the primary maxima. As N increases, the primary maxima increase in intensity as the 1st, 2nd, 3rd, 4th,etc. order spots, and these lesser maxima decrease in intensity until they cease to exist for a grating.

The picture below is for a three slits, so it shows one minor peak between each pair of major peaks.