Waves

Revision notes

Waves transfer energy and information without a net transfer of matter. Transverse oscillations are perpendicular to the direction of energy transfer and longitudinal oscillations are parallel. Amplitude, wavelength, frequency and period describe wave motion. Wave speed is v = fλ and period is T = \(\dfrac{1}{f}\). The speed of sound in air can be measured with two microphones or an echo, and water-wave speed with a ripple tank. Electromagnetic waves are transverse and form a continuous spectrum from radio waves to gamma rays. They travel at the same speed in a vacuum but have different frequencies and wavelengths. At Higher Tier, differences in absorption, transmission, reflection and refraction with wavelength are considered, including ray and wavefront models of refraction. Electromagnetic-wave interactions with matter determine applications and hazards, including ionising effects at the high-frequency end of the spectrum.

Wave quantities and types

Waves transfer energy without a net transfer of matter from source to destination. Transverse oscillations are perpendicular to the direction of energy transfer; longitudinal oscillations are parallel.

Frequency is the number of complete oscillations each second. Wavelength is the distance between equivalent points on successive waves. Wave speed is linked by v = fλ, and period is related to frequency by T = \(\dfrac{1}{f}\).

Refraction and wave behaviour

When a wave enters a region where its speed changes, its wavelength changes while the source frequency remains the same. At an angle, the change in speed can also change direction.

Electromagnetic waves

Electromagnetic waves are transverse and can travel through a vacuum, all at the same speed. In order of increasing frequency (decreasing wavelength) the electromagnetic spectrum is: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Higher-frequency radiation carries more energy, and ultraviolet, X-rays and gamma rays are ionising.

Practical measurement

Measuring several wavelengths or several oscillations can reduce percentage uncertainty compared with measuring only one. Repeat measurements help identify anomalous data and estimate a reliable mean.

Exam technique

Keep frequency, wavelength and speed distinct. For refraction, state what stays constant and what changes. In calculations, convert wavelength units to metres before using v = fλ.

Reading wave diagrams

Amplitude is measured from the equilibrium position to a crest or trough, not from crest to trough. Wavelength is measured between equivalent points on neighbouring cycles. On time-based traces, the horizontal spacing of cycles gives period rather than wavelength unless the horizontal axis represents distance.

When comparing waves, use the equation and the diagram together. If speed is unchanged, increasing frequency means decreasing wavelength.