An oscillator produces narrow pulses (oscillator set to 1MHz, Div10 for pulses), which are input into the transmission line. The scope is triggered on the initial pulse, and the pulse is displayed on the top trace of the dual trace scope as it enters the transmission line and as it returns. The pulse at the end of the transmission line may be displayed on the bottom trace if desired.

For a short section of transmission line the input and output pulses almost line up. Using about 100 feet of transmission line the delay is approximately 150 ns; for the test cable v/c was approximately 0.67 so the wave time is about 1.5 nanosecond per foot.

To see reflections, use only the top trace. The far end can be left open, shorted, or terminated with a 50 ohm terminator to prevent reflection, as shown in the center two photographs above.

The photographs below these two show the oscilloscope showing the 10 MHz pulse train (left), reflections with the far end of the cable connected to the lower trace of the oscilloscope, with the horizontal scale 250 nanoseconds per division (left center) and 100 nanosecond per division (center), the far end terminated by 50 ohms (right center), and the far end shorted but not connected to the oscilloscope (right).

This demonstration makes an excellent companion to K8-04: Speed of Light which uses a similar technique to time a light pulse reflecting in air.

For engineering physics classes the circuit can also be used as an example of RLC resonance.

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.

Lower the cage over the radio. Once the cage surrounds the radio completely, it prevents RF waves from passing through, silencing the radio. This also shows the wavelength dependence of a Faraday cage; the much shorter wavelengths of the electromagnetic waves of visible light pass through the openings unhindered.

This demonstration uses one of the old overhead transparency projectors that focuses the light by a large parabolic mirror under the platform (rather than a Fresnel lens on the platform as in newer models), as seen in the images above. The heat filter and the mirror system above the projector have both been removed. There is sufficient heat focused about two feet above the projector to burn a piece of black paper in a few seconds. In a dark room, the focal point can be clearly seen as the smoke from the paper scatters the light.

Engagement Suggestions

Invite students to predict what would happen if you used white paper rather than black.

• • Would it still burn?
• • Would it take more or less time to ignite?

Background

This demonstration illustrates two important points. It clearly shows that light can be focused to a point by a curved reflector. It is also an illustration of infrared radiation, and the connection between light and heat. When appropriate to the course, consider combining this with a discussion of the wavelengths of the electromagnetic spectrum, and the relationships of energy, heat, and temperature.