Vibrating tuning fork and rotating mirror

Demonstration

It is worth the effort of setting up this demonstration to project the image of the oscillating tuning fork.

Apparatus and Materials

• Small piece of mirror (5 mm x 2 mm)
• Tuning fork (256 Hz or more)
• Small piece of a thin plane mirror, or aluminized Melinex polyester film (gauge 25) (see Technical notes)
• Motor, suitable to drive the rotating mirror
• Contact adhesive (for example, Evo-stik)
• Compact light source
• Converging lens (+ 7D)
• Screen
• Power supply, low voltage, variable

Health & Safety and Technical Notes

Put a guard around the mirror so it cannot fly off and hit observers.

Read our standard health & safety guidance

A tuning fork with a mirror attached to it does not make a beam of light oscillate enough to show clearly. The motion can be made visible by attaching the mirror to the tuning fork arm. The mirror is then driven by the fork with much the same motion, but larger, and it will deflect a small beam of light with considerable motion. The rotating mirror sweeps the light across a screen.

To set up the apparatus: Carefully secure the small piece of mirror to one limb of the tuning fork [alternatively, secure a small strip of aluminized Melinex to the fork, to act a mirror.]

Firmly clamp the fork in the vertical position.

Using the lens, converge light from the lamp filament to an image on the screen about 2 m away, and then place the tuning fork mirror in this beam about a metre from the lens.

Mount a single plane mirror, so that it can rotate about a vertical axis when driven by the fractional horsepower motor, as near as possible to the fork where it can intercept the reflected beam. The image is received on a screen about a metre away.

Alignment is tricky and needs some playing around with: persevere, possibly setting up one mirror at a time.

Procedure

1. Drive the motor at about 600 rotations per minute. The light spot will be seen to travel across the screen.
2. Pluck the fork vigorously and the light spot will trace a sinusoidal wave form. You may need to adjust the frequency of the rotating mirror so that the trace is repeated in the same position on each rotation.

Teaching Notes

• The experiment shows that a vibrating tuning fork oscillates with a S.H.M. Students should previously have seen the time trace produced by an oscillating pendulum
• This experiment is similar to...

  Musical frequencies shown on a C.R.O.

  ...but demonstrates the nature of a tuning fork’s motion directly. Though the other one is much easier to set up, students may find this experiment easier to understand. The two experiments could be demonstrated one after the other.