10 demos with tuning forks

Top 10 Demonstrations with Tuning Forks [W/Video]

I have been using tuning forks in my classroom for 10 years, and in each of those years I have discovered several new tricks.  I hope you can learn many of these from this publication.  For a more complete treatment and my references, please see my article in “The Physics Teacher” March, 2013.

General Usage

0.     General Usage

When a tuning fork is struck it will vibrate wildly in unintended ways.  Imagine putting your arms straight above your head and clapping.  That is the proper motion of a tuning fork.  The problem is that it is also wobbling at the “elbows.”  You can get rid of the unwanted vibrations by touching gently near the joint after striking the fork.  The vibrating tuning fork should be almost silent when used properly.  Hold the tines near your ear and you will hear it clearly.  It is best to hit the tuning fork on a knee or the ball of your hand, avoiding metal on metal.  This is because when tuning forks become chipped they change their inertia and will vibrate at different frequencies.  Spin the fork as you listen and notice that it is loudest right between the tines.  (Constructive Interference.)

Water Dip

1.      Water Dip

Putting a tuning fork in water is one of the best ways to get students accustomed to handling it.  Give a tuning fork to each student or every other student.  Set out several cups of water.  It is always a surprise to see the splash, students will gasp.  These introductory activities are important for laboratory management because the tuning fork is a fun toy and does require some getting used to; the sensation of hearing the tines vibrating is new and somewhat alarming.  It is also a good idea to have boxes or desks or the whiteboard cleared off for students to place the base of the tuning fork against and cause the vibration to resonate.

Strobe Lights

2.     Strobe Lights

A fun demonstration is to put the tuning fork in front of an adjustable strobe light (or a CRT computer monitor).  The strobe light can be adjusted to make the vibration appear slower or even stop!  This works better on a larger tuning fork.  The flashing must match the frequency of the tines, or be very close.  I found my 100 Hz tuning fork to be 99 Hz after investigation.

This effect comes from the strobe light “animating” the fork slowly through time by only making it visible after almost full cycle has passed.  At that moment, the fork will look as though it has only moved slightly.  The difference between the strobe rate and the tuning fork frequency determines the perceived rate of vibration.  The CRT monitor can also act somewhat like a strobe light, but because of its trace across the screen, it causes a wobbly effects in the vibration.


3.      Oscilloscope

Verifying the frequency of the tuning fork can easily be achieved by using an oscilloscope.  This is done by hooking a speaker removed from its housing to the scope’s leads.  You will need to have a proper connection (usually a BNC connector with probe) to achieve this.  You can also use a microphone.  Hold the tuning fork up to the speaker and adjust the settings.  You can see that the fork’s tone is a pure sine curve.  Compare this with the human voice or other instruments such as flutes and kazoos.  Also, try comparing tuning forks of various frequencies and noting the different periods & wavelengths.


4.       Resonance

Two tuning forks that are the same frequency can be made to resonate audibly if the vibration is loud enough.  For this purpose, I prefer using the large box-mounted versions.  Most large glass or wooden objects will have so many resonance frequencies that any tuning fork will cause them to resonate.  Tuning forks that are not the same frequency will not resonate.   The important phrase to understand is “Forced vibration at natural frequency causes resonance.”  Where “resonance” is high amplitude oscillation.  We all experience resonance when singing in the shower; the longer notes resonate better and it makes our voice sound purer in tone.  Also, when our wheels are not aligned in the car and we drive at the natural frequency of our shock springs the car will resonate up and down – but only at specific velocities.

Sound via Light

5.      Sound via Light

Shine a laser on a solar cell from across the room, hook that solar cell to a set of computer speakers and demonstrate the transmission of sound via electromagnetic waves.  This is analogous to radio signals that we listen to because they are also modulated electromagnetic waves.  The laser’s color doesn’t matter much.  I sometimes add smoke to enhance the demo visually.  You will get a less distorted sound if the fork is further into the beam rather than just barely touching it when vibrating.  Clipping to the speakers may require some trial and error.  The “male end” of a stereo cable has its tip going to the left speaker, the middle ring goes to right, and the inner metal goes to ground.  Clip one end of the solar cell to either left or right, but you must clip the other to ground.  A guitar amp will work fine, probably even better.  Clip similarly to the plug of the guitar cord.


6.     Interference

Demonstrating interference is important because it is a property of all waves.  In this case I am using two close frequency waves to show the phenomena called “beats.”  Beats are sometimes also used to tune musical instruments (see #10).  The beating frequency is the difference between the interfering frequencies, the note you hear is the average of the two original frequencies.

This pattern can also be achieved by taking two identical tuning forks and heating one of them with a fire.  (I demonstrate this in the introduction to the video.)  Be sure to wear a hot glove!  The heat reduces the Young’s modulus (similar to spring constant) of the aluminum and the vibrations no longer match.  You can easily tell the difference even with a non-musical ear.

Measure the Speed of Sound7.     Measure the Speed of Sound

With a tube and some water in a bowl it is easy to measure the speed of sound by resonating it with a tuning fork.  The wavelength of the sound must match the length of the tube, but the whole wave doesn’t have to fit inside for this to happen.  Most commonly, the bottom is sealed and becomes a node (a place where the air can’t move) but the top is open and the air can vibrate liberally (anti-node).  The smallest fraction of a standing wave that can fit in here is a ¼ wavelength.  Multiplying wavelength and frequency gives the velocity of sound, usually within 1% error!  If you don’t have a glass tube, this demonstration can also be done with a graduated cylinder that is being filled with water until resonance is achieved.

graduated cylinder


Smoke and Mirrors
8. Smoke and Mirrors

Reflecting light from the end of a mirrored tuning fork can lead to exciting effects.  It gives us a chance to view the motion of the fork by amplifying it as the reflected light is projected across the room.  In the video, I add smoke to help you see the beam.  Because the tuning fork’s motion is sinusoidal in time, it can be made to trace a nearly perfect sine curve in space when it is rotated smoothly at a point far away.

Lissajous Figures

Lissajous Figures are an old method by which tuning forks were tuned.  Excess fork was shaved off to bring the frequency down.  These days, Lissajous Figures are mostly they are used to analyze electromagnetic oscillations in LRC circuits, but originally they were produced by tuning forks reflecting light that is pointed at two mirror loaded forks vibrating at 90 degree angles.  When the frequencies are in ratio you get a Lissajous Figure.  They come in the shape of donuts, pretzels, fish, and other edible items.  It is best to have the forks close, but the wall far away because that will increase the size of the figures and reduces aiming difficulties.

 Strike a Chord

 9. Strike a Chord

Tuning forks come in various frequencies.  You can use them to inform students that music is a branch of physics.   With help you can create chords or even play songs with your students.  Take time to notice that there are specific ratios between notes that are in harmony.  For example, between G and C there is a 3/2 ratio – this is called a fifth.   Between E and C is a 5/4 ratio – this ratio is used in the C major chord.  And between C and A is a 6/5 ratio which is used in the A minor chord.  All octaves (such as middle C and the next C above middle C) are separated by a doubling of the frequency.  These ratios apply to both scientific and musical tuning fork frequencies and it is a fun game to try to discover them by reading your tuning fork labels.

10. Tuning

Tuning an instrument with a tuning fork can be done in many ways.  Typically, the tuning fork is merely listened to or held to the body of the instrument while it is tuned by ear.  But the fork can also be used to resonate the strings into vibration (if they are already in tune).  A completely different method is to strike the note and listen for beats as the sound from the instrument interferes with the sound from the tuning fork.  As the two are brought into tune, they will beat less and less frequently until they are matched with no beating.


It is important to note that the scientific tuning forks do not match the musical frequencies.  For example, A 440 Hz is a musical note, whereas A 426.7 Hz is the scientific note.  In the figure, my guitar tuner thinks my scientific tuning fork is flat by a half step.  The scientific scale is arranged around middle C being 256 Hz (C is 261.6 Hz on the musical scale).  The setting of the musical scale was done somewhat arbitrarily done by German musicians in the early 20th century.  The scientific scale is convenient where all C notes are a multiple of 2; for example, the first C above middle C is 29=512 Hz.  Many of the other frequencies are also whole numbers, such as G 384 Hz and D 288 Hz.

james_lincoln  James Lincoln

Tarbut V’ Torah High School
Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That”  and  the UCLA Physics Video Project.

Contact: [email protected]

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Cheapie Oscilloscope

from Al Guenther’s Science Solutions

Background: The Light Lever

Many scientific measuring instruments have dials with pointers to indicate the magnitude of the measurements. A pointer is actually a lever which magnifies the distance that the instrument moves. In ordinary clocks, speedometers and electric meters for example, the rotating shafts move very small distances, but the pointers greatly magnify these motions so they are easy to see. Unfortunately, pointers have mass which the instrument must move. In very sensitive instruments this presents a problem, as the force which the instrument measures is too weak to overcome the mass of the pointer.

The solution to the problem is a massless pointer–a beam of light referred to as a light lever! A small mirror is attached to the instrument. A beam of light is reflected off the mirror onto a screen which may be several meters distant. The beam is, in effect, a massless lever several meters long which greatly magnifies a very small movement of the instrument.


Remove both ends of a small steel can such as a soup, nut or single serving juice can. Cut a large round balloon from the mouth, over the top and back down to the mouth. Stretch one of the two halves over one end of the can and tape it securely (see diagram). For additional security it is wise to also wrap tape around the side of the can over the rubber and tape strips.

Use sticky putty (available at stationery stores for temporarily hanging posters) to fasten a small (0.5cm) mirror to the rubber midway between the center and the edge. A plastic mirror is best. The plastic mirror can be cut with a hacksaw or broken into small pieces with pliers (WEAR GOGGLES).

For a more effective oscilloscope cover the end opposite the rubber with a snap-on plastic lid. Cut a hole about 2cm in diameter in the center of the lid.

The laser and the can assembly must be attached to a holder to maintain proper alignment. To make the holder, obtain a piece of corner molding about 25cm long (from a lumber yard). Insert a screw about 10cm from one end (see diagram). Adjust the screw so that the laser beam strikes the mirror and reflects onto a wall.

When the laser and can assembly are properly adjusted, fasten them to the molding with rubber bands or tape. CAUTION: TAKE GREAT CARE TO ASSURE THAT NEITHER THE LASER BEAM NOR ITS MIRROR REFLECTION STRIKES ANYONE’S EYES.


With your mouth close to the open end of the can, make a variety of sounds of varying pitch, ranging from musical notes to raucous noise. You should observe a variety of patterns formed by the laser beam (the light lever) as it reflects from the vibrating mirror onto the shaded surface. CAUTION STUDENTS NOT TO LOOK DIRECTLY INTO THE LASER.

Things to try and notice: How is the pattern different for musical notes and nonmusical noise? Who can make the most regular geometric pattern? How does the distance from the wall affect the pattern? Try different materials in place of the balloon (like plastic wrap). What happens if objects are in the can? Try different sound sources like musical instruments.


  • Small steel can
  • Large round balloon
  • Tape (duct tape or electrical tape is best)
  • Sticky putty
  • Small (0.5cm) plastic mirror
  • Laser pointer
  • Corner molding, 25cm
  • Rubber bands

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Haunted Laboratory: Halloween Physics Part II

Since publishing the last Halloween edition of CoolStuff, we have received many positive comments from teachers who have created their own haunted laboratories. We have learned that teachers not only share their displays with their own ghosts and goblins but often open up their labs to the entire school or community. Therefore, we’ve decided it’s time to revisit the Haunted Laboratory in search of even more science tricks and treats.

This edition of CoolStuff features some new exhibits designed to put a spooky spin on science. The displays, selected with ease of construction in mind, use materials, and apparatus found in most science storerooms. Specialty items, such as a green laser pointer, may be readily obtained from Arbor Scientific. To make the most of your Haunted Laboratory experience, we’re offering the following suggestions from physicist and Haunted Laboratory pioneer Thomas Zepf.

  • The lab should be as dark as possible. Quite often the exhibits provide sufficient ambient lighting. However, safety is always the first concern. If necessary, use additional lighting to ensure that students are able to safely navigate the room.
  • Many pieces of standard laboratory apparatus can be modified for use in a haunted lab. Just let your imagination run wild!
  • To set the mood, decorate the room with a variety of Halloween items. Artificial spider webs, plastic bats, pumpkins, and masks are always good.
  • The overall experience is enhanced by playing Halloween music or recordings of scary sounds. Such recordings are inexpensive and readily available. You may find that your library has a wealth of suitable recordings.
  • Consider using students as facilitators. Enthusiastic and knowledgeable student helpers make the experience more enjoyable and meaningful for visitors. As is always the case, the best way to learn something is to teach it. In addition to serving as docents, students will derive a great deal by assisting in the setup of the laboratory.
  • Consider inviting students from other schools and members of the community to your haunted lab. We are certain you will find the experience gratifying.

(Note: To read Professor Zepf’s account of his experiences with haunted laboratories, see the October 2004 edition of The Physics Teacher magazine.)

Haunted Lab Exhibits

We have all seen a reflection of ourselves in a kitchen or bedroom window at night. Because we are in a room that is well illuminated and the outside is dark, light reflected by the window predominates over the light coming through the window from outdoors. We think of the glass as an ordinary mirror, often forgetting that the people on the outside can see us very clearly. This is essentially the “one-way mirror” effect used in situations where an observer wishes to remain hidden. In police stations and department stores, partially silvered panes of glass behave as mirrors to those on one side. To those in a dark room on the other side, the glass functions as a window.


The device shown above demonstrates that a transparent material may be an effective reflector as well as transmitter of light. It also illustrates that the object and the image produced by a plane mirror are equidistant from the mirror’s surface. These two phenomena make the morphing of two faces possible by simply adjusting the intensity of the light falling on two individuals sitting on opposite sides of the glass. The results of this simple exercise can be very freaky, to say the least! Things can become even spookier if one of the participants wears a Halloween mask.


Image courtesy of Pati Sievert at Northern Illinois University

The apparatus consists of a sheet of Plexiglas and two floodlights controlled by dimmer switches. Users sit on opposite sides of the Plexiglas and adjust each light’s intensity until the image of one person’s face merges with the image of their partner’s face. It should be noted that only one participant at a time is able to observe the effect.

The photos above represent two possible setups: a more polished version of the device (Top image) and a very simple setup (Bottom image). In the simpler device, the glass from an old ripple tank serves as the reflecting surface. Two dimmers (Variacs also work well) are used to control the brightness of two standard light bulbs or two flood lamps. An eerie effect may be achieved by using different colored bulbs (e.g. orange and blue) in each of the sockets.

  1. Lights on apparatus may have to be adjusted so they shine on the students who will be sitting on each side.
  2. Have two students sit, one on each side of apparatus, approximately the same distance away from the apparatus.
  3. Students need to line up their eyes and nose as best as they can.
  4. Have students adjust the lights so that one is relatively bright while the other is dim. Have observers view the results from the bright side. Remind the student sitting on the dim side that they will not observe the effect.
  5. Have students reverse the lighting conditions so the student initially on the dim side will have an opportunity to see the unusual effect.

Monsters of the Deep in a Drop of Pond Water

A drop of pond water at the end of a syringe or eyedropper can be treated as a small spherical lens. The light beam that falls on the drop refracts as it passes through the water-air interfaces at both sides of the droplet. The shadows of the small creatures contained in the suspended drop of pond water are magnified up to 1000 times by the liquid lens.

The top image shows the setup for the Monsters of the Deep using a ring stand to hold and adjust the drip rate of the eyedropper.

The bottom image shows the darkroom results of the various creatures we found in our “Swamp Water”.

Image courtesy of the 2006 Arbor Scientific Staff Science Olympics. Our drop of pond water was full of creatures. It was like a Monster convention!  Because of extreme lighting conditions, it was difficult to do it justice.

This activity was inspired by an article in The Physics Teacher magazine by Gorazd Planinsic.

Water-drop projector Physics Teacher 39, 76 (2001)

Watch the laser light on the screen. You should see the magnified shadow images of microscopic organisms floating and moving within the projected light. Small single-cell animals like paramecium appear as dark spots surrounded with interference fringe contours. Larger animals such as mosquito larvae, Cyclops, or water fleas appear like real monsters on the screen.

Pepper’s Ghost

The one-way mirror (see “Freaky Faces”) has long been used in the performing arts, magic acts, and amusement parks. It was exploited for theatrical purposes in the mid-nineteenth century when J.H. Pepper produced what appeared to be a hovering apparition on a London stage. By angling a large glass plate between the stage and the audience, Pepper produced an image of an actor who seemed to hover over the stage (see figure below). To create the illusion, a well-illuminated actor was located in front of and below the stage, out of the audience’s view. A light source of variable intensity was employed to make “Pepper’s Ghost” come and go at will. A darkened theater insured that unwanted reflections would not reveal the presence of the glass. Similar techniques are currently used in amusement parks and museums to startle and amaze unsuspecting visitors.

You can create a simple version of the Pepper’s Ghost illusion with a sheet of glass or Plexiglas, a candle, a drinking glass and a cardboard box. First, remove the flaps from the mouth of the box. Spray the inside of the box with black paint or line it with black construction paper. Place the sheet of glass or plastic over the mouth of the box. Ideally, the sheet of Plexiglas should be the same size as the opening; however, this is not necessary. Place the box on its side with the mouth of the box facing you. Fill the glass with water and place it inside the box. Now cover the opening with the Plexiglas sheet. You may wish to tape it in position. Finally, place a lighted candle in front of the box so that the candle is in front of the drinking glass and both objects are equidistant from the Plexiglas. Now turn off the room lights. When you light the candle, the flame will appear to be burning in the water! The photos below are physicist Dave Wall’s Pepper’s Ghost device.

A setup resembling the original Pepper’s Ghost apparatus may be constructed from a large sheet of glass or Plexiglas, a light source, and a person in costume or a scary mannequin. The reflecting sheet should be slightly taller than the observer. A smaller sheet may be used by raising it with supports.

A glass of water is placed at the exact location of the reflected image.

Here the actual candle is shielded from the viewer by a partial cylinder.

A costumed person or mannequin stands in front of the Plexiglas. When the person or mannequin is illuminated, a ghostly image will appear behind the clear reflecting surface. It’s fun to watch a friend go behind the Plexiglas and put their arm around the image or pass their hand through the body of the image.

The Reverse Mask

Key Concept:
This exhibit consists of a reverse mask. That is a mask that is intentionally set up to be viewed from the wrong side. The display demonstrates that our eye-brain system is conditioned to interpret all faces as convex even when they are not.

Teacher Instructions:
The centerpiece of this display is a white translucent mask. In the photo, a mask of Einstein is shown (see below for details), but any inexpensive mask will work.  Simply place a standard light bulb behind the reverse mask, stand approximately 10 feet away and view the mask with one eye closed. Presto! The concave face will appear convex! Once the illusion is established, open both eyes. If you are like most people, you will still see a convex face. Now move from side to side. The face will follow you wherever you go!

When we look at a reverse mask, our eye-brain system perceives a normal face for two reasons: shadowing and expectation. We are accustomed to seeing subtle shadowing when human faces are illuminated from above. These shadows tell us that the face is in relief. Similar shadowing is produced when light passes through the translucent mask. Perhaps an even more important factor in establishing a sense of convexity is expectation. Since we virtually never encounter a concave face, our previous knowledge tends to override reality. That is, we tend to see what we believe rather than believe what we see.

As we move past the mask, we see more of one side of the mask than the other. Based on experience, we interpret this change in perspective as being caused by the rotation of the face rather than a change in viewing angle.

If we get too close to the mask, we receive visual information that destroys the illusion of convexity. For example, reflections from the plastic surface often reveal the true curvature of the mask.

Image courtesy of Pati Sievert at Northern Illinois University

Einstein Alive

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Student Instructions:
Stand a few feet away from the mask. With one eye closed, look at the mask. Does the mask appear concave or convex? Now open both eyes. Does the mask continue to look convex? Finally, with both eyes open, move from side to side. Can you escape the gaze of the mask?

Ghostly Messages

Using a little Tide or Cheer and a black light, you can make messages and scary images appear seemingly out of nowhere. Many liquid detergents contain fluorescent dyes. These dyes serve as whitening agents that are intended to make yellowed clothing look white and brighter. When illuminated with ultraviolet light, these liquids fluoresce, giving off a cool blue light. By mixing roughly equal parts of detergent and water, you can produce a “paint” that is invisible in white light but fluoresces dramatically under black (ultraviolet) light. You may write a message or draw with your finger tip or a brush on a tabletop, wall, or sheet of paper. Once it has dried, your painting will be virtually invisible in normal light but readily visible in ultraviolet light.

Image courtesy of Pati Sievert at Northern Illinois University.

The Oozing Flesh Display

This exhibit illustrates an effect often called the “Waterfall” or “motion aftereffect” illusion. If you stare at a waterfall, cells in the eye that sense downward motion becomes fatigued. If you shift your eyes to the side of the waterfall, the nearby rocks appear to float upwards because the cells that sense upward motion are not tired and send a stronger signal to your brain. This causes your brain to conclude that the rocks are moving upwards. The same effect can be observed if you stare at a road while in a moving car and then divert your gaze to a stationary object.

Similarly, eye cells that detect rotation in one direction become fatigued and are overpowered by cells that detect rotation in the opposite direction causing the sensation of reverse movement.



To duplicate this effect, print out the dotted disk and attach it to a slow-moving turntable. An old record player set at 33/13 or 16/2/3 works well. You can get your skin to seem to ooze by following these steps: Stare at the center of the spinning disk for 30 seconds.
After 30 seconds of staring, look at the palm of your hand, face or arm.
Does your flesh seem to be oozing? Does it appear to turn in the same direction as the disk or in the opposite direction?

Try looking at objects in the room after staring at the disk for 30 seconds.

If you don’t have a turntable, mount the disk on a piece of circular cardboard. Then attach the center of the disk to a pencil’s eraser with a thumbtack. Spinning the disk at a fairly constant rate by hand is guaranteed to get your skin crawling!

See more Haunted Physics Lab Exhibits

We started discussing the idea of the Haunted Physics Lab in CoolStuff #11, released October 2003. You can see other haunted exhibits from that issue and much more by visiting the CoolStuff Archives. Every issue is added to the Archives and is searchable by topic, title and volume number.

Did you read Haunted Laboratory: Halloween Physics part 1? if not check it out.

Haunted Laboratory: Halloween Physics part 1