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Activities in High-Speed Photography

 

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Demonstrations of High-Speed Phenomena

Scripts for effective demonstrations of some interesting phenomena are given below.

 

The Demonstrations

  1. Stroboscope and fan
  2. Lifetime of a bursting balloon
  3. Waveforms on a plucked cord
  4. Shape of a racquetball in collision

Demonstration 1. Stroboscope and fan

Cautions:  Since this demonstration uses flashing lights, the audience needs to be warned that flashing lights have been known to cause seizures. Even for people who are not susceptible to seizures, strobe lights can create nausea or headaches, especially when used in a room without ambient light  Having a low light level in the room is helpful but may make the demonstration less effective. In that case, don't leave the strobe on for long periods of time.

 

Equipment:

Electronic stroboscope
Electronic flash unit
Large fan with variable-speed control (blades should be replaced with a large, black cardboard disc)
Post cards or pictures taped to cardboard disc
Post-It note taped to the perimeter of the disc and extending several inches beyond

 

Equipment note: 

The electronic flash unit must be adjusted to give a very short burst of light. This can be achieved by using a flash unit with automatic exposure control.  If, for example, you're using a Vivitar 283, set the dial on the light sensor (labeled auto-thyristor) to yellow. Control of flash duration is described in greater detail here.

 

Method:

  1. Turn the fan on to a medium speed. When the room lights are on, only a blur is seen.
  2. Leave the room lights on and discharge the electronic flash unit toward the fan. The blades can be seen for an instant amidst the blur. Show this several times.
  3. With the room lights still on, turn on the stroboscope, describe what it does and how frequency is controlled. Turn off the room lights and adjust the stroboscope frequency to make the fan blades appear stopped. Ask for an explanation of why the blades appear stopped.
  4. Hold a stick just outside the tips of the fan blades until the Post-It note strikes it repeatedly. The sound of the note hitting the stick repeatedly will be heard even though no apparent contact is seen.
  5. Turn off the fan.

Rationale: This demonstration shows that the blurring that is common when viewing fast-moving objects (rotating, in this case) is due, at least partially, to the lighting under which the objects are viewed. (This may also have something to do with a limitation of the brain to process sequential images when displayed without breaks.) When the same object is viewed under a single, brief flash of light (about 30 millionths of a second in this case), all of its detail can be seen. If the flash is repeated at a rate that is equal to the rotational frequency of the object, the object is seen as if stopped. This is only an illusion, as is evidenced by the Post-It note striking the stick repeatedly.

 

Conjecture: The blurring seen under continuous lighting might also have something to do with a limitation of the brain to process sequential images when those images are displayed without breaks. We know from motion pictures that the brain can process and interpret images of moving objects when the images are flashed rapidly by our eyes as a sequence of individual frames.

 

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Demonstration 2. Lifetime of a bursting of a balloon

 

Equipment:

Electronic flash unit
Sound trigger and cord to flash unit
Balloons and needle

 

Equipment notes:

  1. The electronic flash unit must be adjusted to give a very short burst of light. This can be achieved by using a flash unit with automatic exposure control. If, for example, you're using a Vivitar 283, set the dial on the light sensor (labeled auto-thyristor) to yellow. Control of flash duration is described in greater detail here.
  2. A piezoelectric sound trigger works fine. Here are links on this site to information about sound triggers:

basic information
how to make a sound trigger 
trigger from Radio Shack components

Method:

  1. After seeing Demonstration 1, students should understand why they don’t normally see the rip in a bursting balloon. In order to see the rip, the balloon needs to be illuminated with a flash of light that is significantly shorter than the amount of time it takes a balloon to completely burst apart. Invite a volunteer to hold the flash unit on a balloon. Will he/she be able to discharge the flash unit as the balloon is bursting? It will be apparent to nearly everyone that this can only be done by chance--not by design.
  2. Show and describe a sound trigger and how it works. Have another volunteer hold the sound trigger very close to the balloon as it is being popped. A third volunteer can hold the balloon. Pop several balloons under the same conditions to make sure everyone sees the rip. Invite people to describe what they see.
  3. Move the sound trigger about a meter away and pop the balloon again. Ask for someone to explain why much less of the balloon is visible. Knowing the speed of sound (about 340 m/s), a lower limit for the lifetime of the bursting balloon can be obtained. The time for sound to travel a meter is about 1/340 s. (This would be about 3 milliseconds.) The duration of the burst will be greater than this since it takes a short time for the sound trigger and flash circuitry to respond. (This is probably a few milliseconds.)

Rationale: This demonstration provides a unique and dramatic experience in measuring a very short time interval. There are few if any opportunities in a secondary-school science classroom to make such measurements without the use of electronic timing equipment. The demonstration also provides opportunities--depending on the class and the teacher--to use the definition of speed, do estimation, and deal with metric units of distance.

 

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Demonstration 3. Waveforms on a plucked cord

 

Equipment:

Electronic flash unit
Interrupter photogate and cord to flash unit
Elastic cord

 

Equipment notes:

  1. The electronic flash unit must be adjusted to give a very short burst of light.   This can be achieved by using a flash unit with automatic exposure control.   If, for example, you're using a Vivitar 283, set the dial on the light sensor (labeled auto-thyristor) to yellow.  Control of flash duration is described in greater detail here.
  2. The photogate trigger circuit is the one described in the links below.  The use of an interrupter is highly recommended.  These are available as part of the kit listed below.

basic information
how to make a photogate trigger

  1. The elastic cord can be a length of small diameter shock cord or elastic strips such as those found in fabric stores.  Rubber bands also work, but they're really too short for an effective demonstration.

Method:

  1. Have two volunteers come up to take the ends of the cord and stretch it. Pluck the cord at the center and ask people to describe the blurred waveform that they see. Now ask them to predict what shape the cord would take almost immediately after release, that is, before the cord has time to return to its original, resting position.
  2. Ask for another volunteer to hold the flash unit, aiming it at the cord. Show how the flash unit is set off when something is passed through the photogate. Note that the photogate is composed of two components, an emitter of light and a detector of light. In this case, the light is infrared and invisible to our eyes, similar to that of a TV remote. When light is blocked from reaching the detector, the flash unit is signaled to discharge. Motion detectors operate on similar principles. Some examples are closing elevator doors that sense when someone is going through and the security detector in the corner of the classroom that senses when something is moving in the room.
  3. Stretch the cord upward at the center, ready to be released. Position the interrupter about a third of the way from the pluck point to either end. With the room lights off, release the cord. Have someone describe what they saw. Try again with the interrupter nearer the pluck point. Then try with the interrupter closer and closer to one end of the cord. Note how the trapezoidal shape is retained in all cases.
  4. Repeat step 3 for a pluck point that is much nearer one end than the other. Note that this is the shape to be expected for waves on a plucked string instrument such as a guitar. (The amplitude will, of course, not be nearly as great.)
  5. Note that the trapezoidal or triangular wave shapes are not retained for long as the oscillation of the cord progresses. The curved shape seen under normal room lighting is characteristic of the shape of the cord after it has oscillated many cycles.

Rationale: One purpose of the demonstration is to show a common phenomenon that most people have never seen or suspected. Many people are surprised and fascinated by what they see. The demonstration has value as a lead-in or supplement to a discussion of standing waves in an introductory physics class. As such, it would have more pedagogical value in the spring term when most physics classes are studying waves.

 

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Demonstration 4. Shape of a racquetball in collision

 

Equipment:

Electronic flash unit
Sound trigger and cord to flash unit
Racquetball
Flashlight

 

Equipment notes:

  1. The electronic flash unit must be adjusted to give a very short burst of light.   This can be achieved by using a flash unit with automatic exposure control.   If, for example, you're using a Vivitar 283, set the dial on the light sensor (labeled auto-thyristor) to yellow.  Control of flash duration is described in greater detail here.
  2. A piezoelectric sound trigger may not work well for this demonstration.  A trigger with a wider frequency response is recommended.  The trigger described in FAQ works well.

Method:

  1. This is best done in a room with a high ceiling where the ball can be thrown forcefully against the floor without danger of breaking ceiling tiles or lights on the rebound. Place the flash on the floor pointed at the spot where the ball will hit. People should sit on the floor behind or to the side of the flash so that they are not blinded by it. Place the sound trigger on a carpet scrap about 3 feet from the collision site. Have a volunteer aim the flashlight a distance away so that there's enough light to see by but not enough to overpower the flash.
  2. Throw the racquetball down to the floor or have a volunteer throw it. Ask people to describe what they see from their viewpoint near the floor. Repeat a few times. Then move the sound trigger further and further away to see changes in the shape of the ball during collision.  At some point, perhaps a meter or less away, the racquetball will be seen with an interesting dimpled shape. This shape occurs because parts of the ball near the outer perimeter are in the process of rebounding while the top of the ball is still going down.

Rationale: Like the plucked cord, this demonstration allows people to observe a phenomenon that will be a real discovery for most of them. The explanation is fairly simple in qualitative terms.  For introductory physics students who are used to simplifying the study of colliding objects by treating them as points, the racquetball collisions let them see a real colliding object.

 

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