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Mixing Waves
Bridge Light
This exhibit shows two different light phenomena; color and wave interference. The exhibit consists of a two-tiered table top; the first tier containing a sodium vapor light and the second containing white light. To study the phenomena of color visitors examine a collection of color photographs under both lights and note the differences. Light of different colors is made of waves with different wavelengths. The glowing sodium vapor in the bulb gives off very pure yellow light. In this pure yellow light, all the light waves have very nearly the same wavelength. Under white light, the pigments that make up the colors of a picture reflect different wavelengths so that you see different colors. Under the monochromatic (one color) yellow light, the pigments can only reflect yellow since that's the only light there is. If they absorb the yellow light, they look dark. If they absorb some and reflect some, they look gray.
Piano Strobe
Piano Strobe lets visitors see vibration in action by plucking the strings of an exposed piano soundboard. These strings are usually quite tight when they are strung—allowing for the distinctive note of each string. That tension often means the vibration are too fast to be fully recognized. When visitors pluck a bass string, other harmonically related strings will begin to vibrate. To make this more visible, the strings are illuminated with an adjustable strobe lamp. If the strobe is set to a specific rate, it will appear to freeze the motion of the strings, or appear to drastically slow the apparent motion.
Pipes of Pan
Visitors listen to the background noise through each of ten pipes of varying lengths. When sounds in the room start vibrating the air in the pipes, the air moves down each pipe, then reflects back up again. If the time it takes a wave to make this round trip corresponds to the time between pulses of high pressure, the incoming and reflected waves add together to make a louder sound. The longer pipes reinforce low-pitched, low-frequency sounds, while the shorter pipes reinforce high-pitched, high-frequency sounds.
Resonant Rings
Circular steel bands of various diameters are mounted on a plate which has been cemented to a loudspeaker. By adjusting the frequency of the sound produced by the speaker, a graphic illustration of harmonic resonance can be seen in the rings as they vibrate at various frequencies, and in different modes of vibration. The frequency of the speaker can be read on a digital counter, and exciter levers are provided for two rings so that they may be struck, and their natural frequency observed.
Soap Bubbles
Visitors use large metal hoops to make enormous bubbles. Visitors notice that different sized bubbles wobble at different frequencies. Small bubbles have a small "wobble" frequency while big bubbles have a large "wobble" frequency". Additionally visitors notice that the thin film of soap bubble reflects light as shimmering colors.
Soap Film Painting
Visitors hoist a bar out of a soap solution reservoir creating a large soap film. Beautiful interference colors are observed as the film flows and thins out, eventually turning black due to destructive interference between the film's front and back surfaces. Blowing on the film can set up attractive flow patterns or distort the film into interesting reflective shapes, while shaking the frame can excite various resonant standing waves.
Soap Films
At this exhibit visitors press a button that will dip wire figures into a soap mixture. As the wire figures rise out of the soap solution visitors notice that bands of color form at the top of each soap film. The colors in the band depend on the thickness of the soap film. The band will broaden as the soap solution drains downward. Each successive band indicates that the soap film has changed in thickness by 1/1,000 of an inch. The shimmering colors of a soap bubble are created when white light reflected from the front of the film meets white light reflected from the back of the film. When light shines on the soap film, some light waves reflect from the front surface of the film and some reflect from the back surface of the film. When these two sets of reflected waves meet, they can add together, cancel each other out or partially cancel each other out. The thickness of the film determines which colors will reinforce each other and which will cancel. The interaction of light waves reflecting from thin films also produces the colors that you see in oil slicks, abalone shells, opals and some kinds of feathers. Lenses used in good cameras and telescopes are coated with very thin film to prevent the reflection of light.
Vibrating Strings
Vibrating Strings lets visitors both see and feel the change in waves as they are carried over the length of a string a user holds. A small motor vibrates a string at 120Hz. Standing waves are set up in the string, and nodes and anti nodes appear. If the tension on the string is changed, the number of nodes and anti nodes can be varied. Additionally, the frequency of the vibrating string matches the frequency with which a fluorescent light cycles from blue to yellow light, so one end of the string seems to glow blue while the other end glows yellow.
Violin Patterns
Visitors "play" a round or square steel plate with a special violin bow. The nodes of vibration on the plate, called Chlandi figures, are made visible by sprinkling sand on the plates; the sand dances away from the vibrating areas and settles in the still areas. High pitched tones from the plates are due to short wavelength oscillations, which produce closely spaced patterns of sand.
Visible Effects of the Invisible (currently off exhibit)
A long tube partially filled with kerosene is stimulated with sound from a speaker attached at one end. At certain frequencies, the liquid will squirt up, forming fountains at the anti nodes in the tube. The higher the frequency, the more fountains, forming a spectacular demonstration of resonance in the tube. The loudness and frequency of the sound coming from the speaker may be adjusted.