Engagement Suggestion

Background

A single square array of small lights has a full-silvered mirror in back and a half-silvered mirror in front. A long black box placed in back of the infinity mirror appears to have many rows of lights in it until it is removed!

An interesting sidelight is to use this device to indicate the dynamic range of the eye. Each successive row of lights has an intensity of about 1/2. Approximately twenty rows of lights can be seen by the typical naked eye, so the dynamic range of the eye is at least as great as 2^20, or 1,048,576 to 1.

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.

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.

Three sources are permanently mounted on a roll-around cart, from top to bottom: (1)a clear glass long-filament incandescent light bulb which produces a continuous white light spectrum, (2) a mercury lamp which produces a line spectrum, and (3) a cadmium lamp which produces a line spectrum

These spectra are seen using 1"x2" sections of a large roll of replica diffraction grating material with 13,200 lines per inch. The pieces of grating material are relatively cheap, and may be given to the students. Tell your students to go away and look at the spectra of various lights.

The three lamps are mounted in a vertical line so the colors of the lines are the same as those in the adjacent white light spectrum. Point out that the spectra of mercury and cadmium are very different, and generalize that observation to suggest uniqueness of the spectra for each material.

A selenium photoresistor is in series with the coil in a relay circuit used to turn a light on and off. When no light shines on the selenium, it has a high resistance; however, as light falls on it the resistance falls. When the resistance is low the photoresistor passes current which turns on the relay. The light is connected in series with 110VAC through a normally open relay contact. Thus the light is made to go on and off because of the change in the resistance of the selenium due to an external light, which causes the relay to close or open.

During the daylight, when a streetlight remains on, it is often due to pigeons despoiling the photocell, as illustrated in the photograph at the right above, where a piece of simulated pigeon poop has been positioned on the photocell, blocking the light.