Engagement Suggestion

Background

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.

Project the speckled laser pattern on a screen after expanding the beam to about 5 cm diameter. Remove your glasses for the eye test. Watch the speckle pattern as you move your head from side to side. If the speckles move opposite to the direction of your head motion you are nearsighted; if the speckles move in the same direction as your head motion you are farsighted. Put your glasses back on and repeat the experiment to check the adequacy of your lens prescription.

The eye focuses the speckle pattern at its natural focal point. Parallax between the focal plane of the eye lens and the retina cause the apparent motion of the speckle pattern.

A meter stick is held by one person directly above the hands of a second person, the victim. When the meter stick is dropped, the victim closes its hands to catch the meter stick. The reaction time T of the victim can be calculated as T=SQRT(2S/g), where g is the acceleration of gravity and S is the distance the meter stick has fallen.

The experiment is then repeated with the room darkened. Typically the meter stick will fall considerably further, due to the longer reaction time in a darkened environment. The increased time, due to visual latency, can be determined by subtracting the reaction time in the light from that in the dark.

This is one reason for increased reaction time in night driving.

The trapezoidal window is rotated at about one revolution per two seconds by an electric motor in the box. When the taller side rotates away from us, it appears to reverse direction and rotate toward us.

Everyone knows that any object appears to get smaller as it moves away and bigger as it moves toward you. Mind over matter!

The classic impossible triangle is constructed from three orthogonal aluminum bars. The front bar has a cutout which is aligned with the rear bar so that the triangle appears closed when viewed from a particular point (or along a short line), forming an impossible figure.

This gizmo bothers people more when it is set up with a pendulum apparently swinging through the "solid" metal of the triangle. The photograph shows the triangle with the pendulum at its equilibrium position along with the view of the triangle from an appropriately positioned camera.

Photographs of actual 3D realizations of a number of Escher art works will be found at the "Escher for Real" web site http://gershonelber.org/EscherForReal/.

Note: Due to complexities of setup, please give at least three working days notice when ordering this demonstration.

A set of liquid vials containing fluorescent materials is illuminated by an ultraviolet light source. The radiation is absorbed by the chemicals and emitted at a visible frequency, causing them to glow. Identification of the chemical is printed above each vial. The bottle at the right contains quinine water. It's almost enough to make you give up drinking quinine water.

The second photograph was taken with normal fluorescent lighting.