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PiP Oct 2013

  • G3-28 SUSPENDED SLINKY

    G3-28
    Shows longitudinal and transverse traveling waves & standing waves
    Transverse or longitudinal pulses can be created by appropriate motion of your hand at one end of the SLINKY. Using your hand you can also create transverse standing waves and discuss the overtone series. Gently vibrating one end of the spring (either by hand or using the motor) at the appropriate frequency creates longitudinal standing waves.
    FS1

  • H1-01 BELL IN VACUUM

    H1-01
    Demonstrates sound wave requirement for a medium

    An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

    Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

    Engagement Suggestion
    • Consider asking the students to make predictions before each step - how will removing the air change what they hear? What they see? What will happen as the air returns?
    • Compare this to videos the see of people working in the vacuum of space, in real life and in the movies. What do you see and hear in real life? How is this presented in fiction, and why?
    Background
    There are subtleties to this effect. The pump is not creating a true vacuum within the chamber. The vast majority of the air has been removed, reducing the environment’s ability to transmit sound; but the other (perhaps more important) effect in play is the difference in density between the interior of the chamber and the glass and the external atmosphere; this creates a major change in impedance, causing what little sound can be transmitted within the chamber to reflect back. Also, off course, the bell is not floating in free space, and some vibrations can always be transmitted through the supports and wires.

    For small groups, also consider H1-04, a more portable version of this demonstration.
    FS1

  • H1-02 SPEAKER AND CANDLE

    H1-02
    Demontrates longitudinal behavior of sound waves
    A lighted candle is placed directly in front of the center of a large loudspeaker, which is operating in the 10 Hertz range. The motion of the candle flame is longitudinal, following the motion of the air, illustrating the longitudinal nature of sound waves.

    With a bit of exploration, one can find resonances in the system that produce the most dramatic flame displacement. Consider having students make predictions about how different waveforms will make the flame respond differently
    OS5, ME2

  • H2-21 AUDIBLE YOUNG'S EXPERIMENT - GROUP LISTENING

    H2-21
    Demonstrates interference of sound waves with two coherent sources
    The oscillator-amplifier is set to approximately 3000 Hz, with identical signals being applied to both loudspeakers. Rotating the loudspeakers past the listeners allows them to observe the interference pattern by hearing the alternating maxima and minima in the intensity pattern.
    OS2

  • H2-32: SPEAKER WITH BAFFLE

    H2-32
    Demonstrates diffraction and interference of sound waves

    A small loudspeaker plays music with lots of bass, but the bass is not very loud. When the speaker is held up behind a hole the size of the speaker in a board about two feet square, the sound becomes much louder to the audience; this is particularly noticeable in the lower (bass) frequencies.
    Background
    A loudspeaker produces two distinct sound waves: one from the front and one from the back, which are out of phase with respect to each other. In the absence of the baffle, these sounds both diffract in all directions, and, because they are exactly out of phase they interfere destructively, especially the bass. The baffle forestalls the diffraction and thus reduces the magnitude of the interference. This effect is used in constructing speakers and their enclosures, to ensure that the maximum of output energy is passed to the listener. It can also be observed in nature, as some insects have been noted to use such surfaces to effectively amplify their calls in the wild (see references below).
    H2

  • H2-41 DOPPLER BALL

    H2-41
    Demonstrates Doppler effect

    An electronic device making a loud squeal is turned on and placed inside a foam ball. The ball is then zipped inside a cloth cover hooked to the end of a cord, and whirled about the instructor's head or carefully tossed from person to person. The Doppler effect can easily be heard throughout even a large room.
    Engagement Suggestion:
    • Challenge students to describe other circumstances where they have heard this phenomenon
    Background:

    This is a classic illustration of the Doppler Effect. When a wave source is in motion, the wavelength of the emitted waves is observed to change by an observer along its direction of motion.

    It can be useful to present this in conjunction with an animation or simulation, to illustrate the effect visually; see the relevant page of our Directory of Simulations.
    H2

  • K8-01 ELECTROMAGNETIC WAVE - MODEL

    K8-01
    Shows the relationship between the electric and magnetic field vectors in a plane-polarized traveling electromagnetic wave
    Red pegs represent the electric field vector and blue pegs represent the magnetic vector. The spatial relationship between these vectors and the direction of propagation can be seen. By moving the model along its axis the temporal aspect of the wave can be shown. This wave has a wavelength of 0.81 meters, and as an EM wave would have a frequency of 370MHz
    FS1

  • L3-16 FOCUSING OF HEAT WAVES BY MIRRORS

    L3-16
    Demonstrates that concave mirrors can focus heat waves
    Two parabolic concave mirrors are used to focus heat from a nichrome heater and light a match.
    L3, PW1

  • L4-01 OPTICAL BOARD - RECTANGULAR SLAB

    L4-01
    Demonstrates refraction. Shows displacement of rays in a uniform slab of glass
    A slit baffle with concave and convex mirrors are used to produce a beam of parallel rays of light. A rectangular slab of lucite placed at an angle in the rays of light produces refraction at each surface, leading to displacement of the light rays. The central ray in the picture is reflected internally off the end surface of the slab and directed upward.
    FS0

  • L5-12 PLEXIGLASS SPIRAL

    L5-12
    Demonstrates total internal reflection
    Due to total internal reflection the light from the lamp remains mostly confined within the spiral plexiglass rod, and only exits at the end, where the angle between the surface and the incoming light exceeds the critical angle

  • M7-03 TWO POLAROIDS AND LIGHT SOURCE

    M7-03
    Demonstrates polarization of light
    The first polaroid circle polarizes the light. Rotating the front polaroid causes the light to become alternately brighter (polaroids aligned) and dimmer (polaroids crossed). This is best performed with a semi-diffuse light source, such as an incandescent lightbox.
    M7, LS1
    Polarization

  • M8-01 POLAROIDS AND KARO SYRUP

    M8-01
    Demonstration of an optical cavity
    Place a glass bottle of Karo syrup between two crossed polaroids lighted from behind, then rotate one of the polaroids. The second polarizing sheet removes a small band around one wavelength of light, to produce negative colors.
    M8, M7, LS1