### A strobe light can make a vibrating string appear bent and unmoving in midair!

WATCH THE VIDEO

Other Demos of Interest:

 Normal Modes of Standing Waves Standing Waves and Resonance Strobe Effect

Teachable Topics
• Strobe Effect
• Optics
• Frequency

Theory:

This demonstration shows the strobe effect. The strobe effect occurs whenever an object in continuous motion - like the vibrating string in this demo - is represented by a series of small sections, much like how videos are just many pictures strung together. In order for the strobe effect to be apparent, the rate at which these 'pictures' are being taken must be close to the rate at which the continuous motion is happening, like the strobe light flashing at the same rate as the string is vibrating.

In this demonstration, a standing wave is formed along the string. By vibrating at the right frequency, the waves sent down the string will interfere with each other to form the familiar wave pattern seen in the video. In the video the n = 4 mode was shown, and the frequency was 56.1 Hz. When the strobe light flashes at the same frequency, the string looks like it is not moving anymore. This is because when the strobe light flashes, the string is at the same point of oscillation each time, causing the string to look like it stationary.

When the frequency of the strobe light is slightly longer than the frequency of the string, the strobe light begins to light up a later and later sections of the string's oscillation period each time it flashes. This results in the string looking like it is slowly oscillating. In contrast, when the frequency of the strobe light is slightly faster than the frequency of the string, the strobe begins to light up the string at earlier parts of its oscillation, causing the string to look like it is slowly oscillating in reverse.

Apparatus:

• Taut length of string
• Pulley
• Sine wave generator
• Mechanical wave driver
• Strobe light with adjustable frequency

Procedure:

1. Set up the apparatus like it was seen in the video. Using the wave generator, produce a standing wave along the string.
2. Turn on the strobe light and adjust the frequency until the string appears to be unmoving, bent around in midair.
3. Try adjusting the frequency slightly up or down. This should result in the string appearing to bend in midair very slowly.
4. Try experimenting some more by changing the frequency of the strobe light and see what happens!