OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(standing) waves using microwaves, stretched strings and air columns
Full specification wording
Stationary waves: (standing) waves using microwaves, stretched strings and air columns
Stationary waves form by the superposition of two progressive waves of the same frequency travelling in opposite directions.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Stationary waves form by the superposition of two progressive waves of the same frequency travelling in opposite directions. With microwaves, a transmitter directs waves at a metal plate; the reflected wave superposes with the incident wave, and a probe detects nodes and antinodes spaced λ/2 apart. On a stretched string fixed at both ends, boundary conditions dictate that nodes form at the ends. The allowed wavelengths depend only on the string's length (L) and harmonic number (n), given by λ = 2L/n. The string's tension and mass per unit length determine the wave speed and frequency, but not the wavelength of a given mode. In air columns, closed ends form displacement nodes and open ends form antinodes.
What earns the marks
- Stationary waves form when incident and reflected waves of the same frequency superpose.
- In a microwave experiment, a metal reflector creates the reflected wave, and a detector locates nodes and antinodes spaced λ/2 apart.
- For a stretched string fixed at both ends, the allowed wavelengths are determined by the string length and harmonic number (λ = 2L/n).
- The tension and mass per unit length of a string affect the wave speed and frequency, but do not change the wavelength of a specific harmonic mode.
- In air columns, a closed end acts as a displacement node and an open end acts as a displacement antinode.
Common mistakes and how to put them right
Stating that the wavelength of a stationary wave on a fixed string depends on tension.
Put it right: Wavelength is fixed by the string length and harmonic; tension changes the wave speed and frequency.
Assuming the distance between adjacent nodes is a full wavelength.
Put it right: The node-to-node distance is exactly half a wavelength (λ/2).
Confusing displacement and pressure nodes in air columns.
Put it right: A closed end is a displacement node but a pressure antinode.
Exam tips
- When calculating wavelength from a microwave interference pattern, remember to double the distance between adjacent nodes.
- For string experiments, use λ = 2L/n to find wavelength, then apply v = fλ to link it to wave speed and frequency.
- Clearly state the boundary conditions (e.g., nodes at fixed ends) when explaining the formation of stationary waves on a string.
Try the lesson
The start of the Stationary waves: (standing) waves using microwaves, stretched strings and air columns lesson, made from checked teaching notes for this specification point.
Listen
Stationary waves form by the superposition of two progressive waves of the same frequency travelling in opposite directions. With microwaves, a transmitter directs waves at a metal plate; the reflected wave superposes with the incident wave, and a probe detects nodes and antinodes spaced λ/2 apart. On a stretched string fixed at both ends, boundary conditions dictate that nodes form at the ends. The allowed wavelengths depend only on the string's length (L) and harmonic number (n), given by λ = 2L/n. The string's tension and mass per unit length determine the wave speed and frequency, but not the wavelength of a given mode. In air columns, closed ends form displacement nodes and open ends form antinodes. A strong answer earns marks for these points. Stationary waves form when incident and reflected waves of the same frequency superpose. In a microwave experiment, a metal reflector creates the reflected wave, and a detector locates nodes and antinodes spaced λ/2 apart. For a stretched string fixed at both ends, the allowed wavelengths are determined by the string length and harmonic number (λ = 2L/n). The tension and mass per unit length of a string affect the wave speed and frequency, but do not change the wavelength of a specific harmonic mode. Exam tip: When calculating wavelength from a microwave interference pattern, remember to double the distance between adjacent nodes.
Key words
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Stationary waves
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Microwave reflection and detection
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Boundary conditions for stretched strings
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Displacement nodes and antinodes in air columns
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See every definition
- Stationary waves
- Formed by the superposition of two progressive waves of the same frequency travelling in opposite directions.
- Microwave reflection and detection
- A transmitter directs waves at a metal plate; the reflected wave superposes with the incident wave, and a probe detects nodes and antinodes spaced λ/2 apart.
- Boundary conditions for stretched strings
- On a stretched string fixed at both ends, boundary conditions dictate that nodes form at the ends.
- Displacement nodes and antinodes in air columns
- In air columns, closed ends form displacement nodes and open ends form antinodes.
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