Progressive wave
Waves - OCR A-Level Physics
Key Definition
Progressive wave
A wave that transfers energy from one place to another without transferring matter. Each particle vibrates about its equilibrium position.
A wave that transfers energy from one place to another without transferring matter. Each particle vibrates about its equilibrium position.
- In a transverse waveA wave where the oscillations are perpendicular to the direction of energy transfer (e.g. light, water waves)., the oscillations are perpendicular to the direction of energyThe capacity to do work. Measured in joules (J). transfer (e.g. light, water surface waves, waves on a string).
- In a longitudinal waveA wave where the oscillations are parallel to the direction of energy transfer (e.g. sound)., the oscillations are parallel to the direction of energyThe capacity to do work. Measured in joules (J). transfer (e.g. sound, ultrasoundSound waves with frequencies above 20 kHz (beyond human hearing). Medical ultrasound typically uses 1-15 MHz., P-waves).
- Longitudinal waves consist of alternating regions of compressionA region in a longitudinal wave where particles are pushed closer together, creating higher pressure. (particles close together) and rarefactionA region in a longitudinal wave where particles are spread further apart, creating lower pressure. (particles spread apart).
- All electromagnetic waves are transverse. Sound waves are always longitudinal.
Key Definition
Amplitude (A)
The maximum displacement of a particle from its equilibrium position. Measured in metres (m).
The maximum displacement of a particle from its equilibrium position. Measured in metres (m).
Key Definition
Wavelength (λ)
The minimum distance between two points oscillating in phase (e.g. crest to crest, or compression to compression). Measured in metres (m).
The minimum distance between two points oscillating in phase (e.g. crest to crest, or compression to compression). Measured in metres (m).
Key Definition
Frequency (f)
The number of complete oscillations passing a given point per unit time. Measured in hertz (Hz), where 1 Hz = 1 s⁻¹.
The number of complete oscillations passing a given point per unit time. Measured in hertz (Hz), where 1 Hz = 1 s⁻¹.
Key Definition
Period (T)
The time taken for one complete oscillation. Measured in seconds (s).
The time taken for one complete oscillation. Measured in seconds (s).
$$T = \frac{1}{f}$$
Key Definition
Phase difference
A measure of how much one wave lags behind another. Expressed in degrees (0° to 360°) or radians (0 to 2π). Two points one full wavelength apart are in phase (phase difference = 0 or 2π).
A measure of how much one wave lags behind another. Expressed in degrees (0° to 360°) or radians (0 to 2π). Two points one full wavelength apart are in phase (phase difference = 0 or 2π).
$$\Delta\phi = \frac{2\pi \, \Delta x}{\lambda}$$
Common Mistake
MEDIUM
Students often: Reading wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). from a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).–time graph instead of a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).–distance graph.
Instead: WavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). comes from a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).–distance graph (snapshot of all particles at one instant). PeriodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s). comes from a displacement–time graph (one particle over time). Always check the x-axis label.
Instead: WavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). comes from a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).–distance graph (snapshot of all particles at one instant). PeriodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s). comes from a displacement–time graph (one particle over time). Always check the x-axis label.
Examiner Tips and Tricks
- When defining wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)., always say 'minimum distance between two points in phase' - not just 'crest to crest'.
- OCR mark schemes require the general definition.