Loading…
Loading…
Loading…
OCR GCSE Physics A · J249
OCR J249 · P5.1b Check the specification (PDF) (opens in a new tab)
A wave is a travelling disturbance that transfers energy. If it travels through matter, the particles of the medium vibrate around their positions rather than travelling along with the wave. Because the disturbance repeats, we can describe its pattern in space using and its rate of repetition in time using .
The wavelength is the shortest distance between two consecutive points that are at the same stage of a wave cycle. Its symbol is the Greek letter lambda, , and its SI unit is the metre (m). Smaller units such as centimetres or millimetres may also be used.
On a transverse-wave diagram, one wavelength can be measured from one crest to the next crest or from one trough to the next trough. More generally, it can be measured between any neighbouring matching points on the wave. The measurement must be made in the direction in which the wave travels. The vertical distance from the middle position to a crest is amplitude, not wavelength.
Longitudinal waves do not have crests and troughs. Instead, they contain compressions, where particles are closer together, and rarefactions, where particles are farther apart. One wavelength is the distance from the centre of one compression to the centre of the next compression, or from one rarefaction to the next.
Wavelength is one complete spatial cycle; frequency is the number of complete cycles occurring each second.
The frequency of a wave is the number of complete waves or oscillations produced or passing a fixed point each second. Its symbol is and its unit is the hertz (Hz).
A frequency of 5 Hz means that five complete cycles occur each second. Imagine watching one fixed point on a rope: if that point completes five full up-and-down oscillations in one second, the wave has a frequency of 5 Hz. Count complete cycles—not individual crests and troughs. One crest and the following trough together still represent only part of a cycle.
The vibrating source sets the frequency. For example, if the end of a rope is moved through ten complete oscillations each second, it produces a wave with a frequency of 10 Hz.
Wavelength and frequency describe different features. Wavelength is a distance, whereas frequency is a number of cycles per second.
A graph showing displacement against distance is a snapshot of the wave along the medium, so the horizontal spacing of one cycle is wavelength. A graph showing displacement against time records the motion at one position, so the horizontal spacing of one cycle is a time interval called the period—not a wavelength. Always inspect the horizontal-axis label before interpreting the separation between peaks.
When identifying wavelength on a diagram, measure parallel to the direction in which the wave travels—not the height of the wave.
Check the horizontal axis before interpreting a graph. On a distance graph, one cycle spans a wavelength; on a time graph, one cycle spans a period.
A crest followed by a trough is only half a complete transverse wave, not one wavelength.
For a longitudinal wave, measure between the centres of neighbouring compressions or neighbouring rarefactions, not from a compression to the next rarefaction.
State frequency in hertz (Hz), where 1 Hz means one complete cycle per second.
It can see this note and what your specification asks for.
Answers are free with an account.
1 free note left this week
Get more free notesYour answer sets when this note comes back for review, or marks it complete.
Was this note helpful?
Log in to tell us whether it helped, or to report a problem.
Last updated
Wavelength
The shortest distance between two consecutive points on a wave that are at the same stage of a cycle, measured in the direction of wave travel.
Frequency
The number of complete waves or oscillations produced or passing a point each second.
Hertz
The unit of frequency; one hertz means one complete cycle per second.
Cycle
One complete repetition of a wave or vibration.
Put your knowledge into practice — try past paper questions for Physics A
Wavelength
The shortest distance between two consecutive points on a wave that are at the same stage of a cycle, measured in the direction of wave travel.
Frequency
The number of complete waves or oscillations produced or passing a point each second.
Hertz
The unit of frequency; one hertz means one complete cycle per second.
Cycle
One complete repetition of a wave or vibration.