Exploring Standing Waves – Using a Strobe to See the Pattern
Standing waves are a perfect example of physics that you can both see and measure. When a vibrating string is viewed under a strobe light, the motion appears frozen, revealing the hidden structure of the wave — the nodes, antinodes, and harmonics that define resonance.
The Experiment
Using a string is stretched under tension and driven by a variable-frequency signal generator.
A strobe light flashes at adjustable frequency to make the oscillating string appear stationary.
As the driving frequency changes, students observe:
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No stable pattern at most frequencies.
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Clear standing waves at resonant frequencies — the string divides into distinct loops separated by nodes.
By synchronising the strobe frequency to match the wave’s motion, students can slow or freeze the pattern, making it easy to count loops and measure wavelength.
The Science
Standing waves form when two waves of the same frequency and amplitude travel in opposite directions, interfering to create stationary nodes and vibrating antinodes.
The condition for resonance is:
where is the harmonic number, is string length, is tension, and is mass per unit length.
The strobe effectively samples the motion at discrete times, creating the illusion of a stationary or slowly moving pattern — allowing students to measure details that are otherwise too fast for the eye to follow.
Skills Highlight
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Using a strobe to visualise rapid oscillations
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Measuring wavelength, frequency, and tension relationships
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Identifying harmonics and verifying wave equations
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Relating resonance to real-world systems such as strings, bridges, and air columns
Why It Works in Teaching
A strobe light turns invisible vibration into visible form. Students can pause motion, count nodes, and verify equations that describe wave behaviour. It’s one of the most satisfying demonstrations of resonance, combining precision measurement with a memorable visual experience.
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