Cambridge O Level Physics · Syllabus 5054 · Waves
Wavelength
What is Wavelength?
Wavelength is the distance between two consecutive identical points on a wave, such as two consecutive crests; its symbol is lambda and its SI unit is the metre.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Wavelength in context
A wave is a disturbance that travels, carrying energy away from a source while the material it passes through only oscillates about fixed positions. Describe the disturbance with wavelength, frequency and amplitude; link them with \(v = f\lambda\). Every wave can reflect, refract and diffract. Light is an electromagnetic wave, so mirrors, blocks, prisms, fibres and lenses are all the same three behaviours applied to rays. Sound is a longitudinal mechanical wave, so it needs a medium, its pitch follows frequency and its loudness follows amplitude, and its echoes measure distance.
A wave is a disturbance that travels through space or a material, transferring energy from a source to a receiver without transferring matter. The particles of the medium oscillate about fixed equilibrium positions and end up where they began. Describe the wave with its wavelength, frequency and amplitude, and link them to the speed with \(v = f\lambda\). Waves of every kind can be reflected at a plane surface, refracted when their speed changes, and diffracted at a gap or an edge.
The electromagnetic spectrum is one continuous family of transverse waves that all travel at \(3.0 \times 10^{8}\) m/s in a vacuum (and at approximately the same speed in air), transfer energy, and need no material medium. In order of increasing frequency and therefore decreasing wavelength the main regions are: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Each region is used for the jobs its particular frequency and wavelength suit, and the hazards rise sharply at the high-frequency end, where the radiation becomes ionising.
Common mistakes with Wavelength
- 4. “Frequency changes when a wave crosses a boundary.” Why wrongIf fewer crests left the boundary each second than arrived at it, crests would have to pile up there without limit. They do not. CorrectFrequency is fixed by the source. Speed changes at the boundary, and since \(v = f\lambda\) with \(f\) fixed, the wavelength changes in the same ratio. Say this“The frequency is unchanged because it is determined by the source; the speed and wavelength both change.” CheckA 500 Hz sound passes from air into water where it travels about four times faster. State what happens to \(f\) and to \(\lambda\).
- 6. “Diffraction is greatest through a very wide gap.” Why wrongIt confuses how much energy gets through with how much the wave spreads. A wide gap passes plenty of energy, almost straight on. CorrectSpreading depends on the ratio of wavelength to gap width and is greatest when the gap is comparable to the wavelength. Say this“Diffraction is most noticeable when the gap width is about the same as the wavelength.” CheckWaves of wavelength 2 cm meet a gap. Which gives more spreading, 2 cm or 20 cm?
- 10. “Light always bends when it crosses a boundary.” Why wrongBending needs an oblique wavefront so that one end slows before the other. A wavefront arriving square-on slows all at once. CorrectAt normal incidence (\(i = 0^\circ\)) the speed and wavelength change but the direction does not. Say this“A ray travelling along the normal is not deviated, although its speed and wavelength do change.” CheckWhy is a ray aimed at the centre of the flat face of a semicircular block not bent at the curved surface?
Questions students ask about Wavelength
Why does the frequency of a wave stay the same when it crosses into a different medium?
Frequency is fixed by the source, not by the medium the wave is travelling through. If fewer wave crests left a boundary each second than arrived at it, crests would pile up there without limit, which does not happen. Crossing into a new medium changes the wave's speed, and since \(v = f\lambda\) with \(f\) fixed, the wavelength changes in the same ratio as the speed.