What are transverse and longitudinal waves, and how are wave speed, frequency and wavelength related?
Transverse and longitudinal waves, the meaning of amplitude, wavelength, frequency and period, the wave speed equation, the relationship between frequency and period, and the reflection and refraction of waves.
A focused answer to the OCR Gateway GCSE Combined Science A topic P4 on wave properties, covering transverse and longitudinal waves, amplitude, wavelength, frequency and period, the wave speed equation, the link between frequency and period, and reflection and refraction.
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What this topic is asking
OCR wants you to distinguish transverse and longitudinal waves, define amplitude, wavelength, frequency and period, use the wave speed equation, relate frequency and period, and describe reflection and refraction.
Transverse and longitudinal waves
All waves transfer energy (and information) without transferring matter: the particles or fields oscillate about a fixed point but do not travel along with the wave. A floating object on water bobs up and down as a wave passes but does not move along with it, which shows that it is energy, not matter, that is carried. Sound is a longitudinal wave that needs a medium (it cannot travel through a vacuum), while electromagnetic waves are transverse and can travel through a vacuum.
Describing a wave
A bigger amplitude means more energy (a louder sound or a brighter light); a higher frequency means a shorter wavelength for the same wave speed. The period and frequency are reciprocals of each other, so a wave with a short period has a high frequency.
The wave speed equation, reflection and refraction
The speed of a wave is linked to its frequency and wavelength by the wave speed equation:
(wave speed in m/s frequency in Hz wavelength in m). You can measure the speed of a wave in the required practical, for example by finding the frequency and wavelength of waves on a string or ripples in a tank. Waves can also change direction at a boundary:
- Reflection is when a wave bounces off a surface (such as an echo of sound, or light reflecting from a mirror); the angle of incidence equals the angle of reflection, both measured from the normal (a line drawn at right angles to the surface).
- Refraction is when a wave changes direction as it passes from one medium into another because its speed changes (for example light bending as it enters glass), which happens when the wave meets the boundary at an angle. The wave slows down and bends towards the normal entering a denser medium, and speeds up and bends away from the normal leaving it; its frequency stays the same, so its wavelength changes.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA wave has a frequency of 50 Hz and a wavelength of 6 m. Calculate its speed, and state the difference between a transverse and a longitudinal wave.Show worked answer →
A Physics Paper 6 calculation plus recall. Method: wave speed m/s (frequency times wavelength). Markers credit the correct substitution and the answer with units. The difference: in a transverse wave the oscillations (vibrations) are at right angles (perpendicular) to the direction the wave travels, for example water waves and electromagnetic waves; in a longitudinal wave the oscillations are parallel to the direction of travel, with compressions and rarefactions, for example sound waves. Markers want the correct wave speed and a clear statement that transverse vibrations are perpendicular and longitudinal vibrations are parallel to the direction of travel.
OCR 20214 marksA wave has a period of 0.02 s. Calculate its frequency, and then calculate the wavelength if the wave travels at 340 m/s.Show worked answer →
A P4 two-step calculation. Method: frequency is one divided by the period, Hz. Then rearrange the wave speed equation for wavelength: m. Markers credit using to get Hz, and rearranging correctly to get a wavelength of m. A common error is to forget to invert the period for frequency, or to multiply rather than divide when finding the wavelength.
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