What are transverse and longitudinal waves, and how do they differ?
Transverse and longitudinal waves: how each type oscillates relative to the direction of energy transfer, and examples of each.
A focused answer to AQA GCSE Physics 4.6.1, covering the difference between transverse and longitudinal waves, how the oscillations relate to the direction of energy transfer, and examples including light, sound and water waves.
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What this dot point is asking
AQA wants you to describe transverse and longitudinal waves, explain how the oscillations relate to the direction of energy transfer, and give examples of each, including that waves transfer energy but not matter. This is part of topic 4.6.1 of the AQA GCSE Physics (8463) specification.
Waves transfer energy, not matter
This is one of the most important ideas about waves and is tested in many forms. The energy carried by a sound wave can rattle a window, and the energy carried by light from the Sun warms the Earth, yet in neither case does matter travel all the way from the source to the receiver: the air does not blow from the loudspeaker to your ear, and no particles cross the vacuum of space from the Sun. Instead, each part of the medium (or, for light, the electric and magnetic fields) disturbs the next, passing the energy along while the matter stays roughly where it was. A field of wheat in the wind shows the same effect: a wave appears to ripple across the field, but each stalk only sways back and forth and stays rooted in place.
Transverse waves
A transverse wave has visible peaks (crests) and troughs. A useful way to demonstrate one is to flick the end of a long rope or stretched spring up and down: a wave runs along the rope, but any one point on the rope only moves up and down, not along, which shows the oscillation is perpendicular to the direction the wave travels. All electromagnetic waves are transverse, which is an important fact to recall because it is one of the shared properties of the entire electromagnetic spectrum.
Longitudinal waves
A longitudinal wave can be shown by pushing and pulling the end of a stretched spring (a slinky) along its length: a pattern of compressions and rarefactions travels along the spring, while each coil only moves a little back and forth about its own position. In a sound wave the air particles do the same thing, bunching together into compressions and spreading apart into rarefactions as the wave passes. For a longitudinal wave the wavelength is the distance from one compression to the next, just as for a transverse wave it is the distance from one crest to the next.
Try this
Q1. State the difference between a transverse and a longitudinal wave. [2 marks]
- Cue. Transverse: oscillations perpendicular to energy transfer; longitudinal: oscillations parallel to energy transfer.
Q2. Give one example of a transverse wave and one example of a longitudinal wave. [2 marks]
- Cue. Transverse: light (or water ripples); longitudinal: sound.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe the difference between a transverse wave and a longitudinal wave, giving one example of each, and explain what is meant by the statement that a wave transfers energy but not matter.Show worked answer →
In a transverse wave the oscillations are at right angles (perpendicular) to the direction in which the wave transfers energy, for example light or any electromagnetic wave, or ripples on water (1 mark for the description, 1 mark for a valid example). In a longitudinal wave the oscillations are parallel to the direction of energy transfer, producing compressions and rarefactions, for example sound (1 mark covering description and example). A wave transfers energy but not matter because the particles (or fields) of the medium only oscillate back and forth about a fixed position and return to where they started; they do not travel along with the wave, so energy is passed on while the matter stays put (1 mark). Markers reward the perpendicular-versus-parallel distinction, correct examples, and the oscillation-about-a-fixed-point explanation.
AQA 20213 marksA cork floats on the surface of a pond. Explain what happens to the cork as water ripples (transverse waves) pass beneath it, and use this to explain why a water wave does not carry the water across the pond.Show worked answer →
As the ripples pass, the cork bobs up and down (oscillates) about a fixed position on the surface, moving perpendicular to the direction the wave travels (1 mark). The cork does not move across the pond with the wave; it returns to roughly the same place after each ripple passes (1 mark). This shows that the water itself is not carried across the pond, because the water (like the cork) only oscillates up and down about a fixed position while the wave transfers energy across the surface (1 mark). Markers reward the up-and-down oscillation of the cork, that it stays in roughly the same place, and the conclusion that the medium is not transported, only energy.
Related dot points
- Properties of waves: amplitude, wavelength, frequency and period, the wave speed equation, and the required practical for measuring wave speed.
A focused answer to AQA GCSE Physics 4.6.1, covering the amplitude, wavelength, frequency and period of a wave, the wave speed equation, the period equation, and the required practical for measuring the speed of waves.
- The electromagnetic spectrum: the order of the seven groups, their shared properties, their uses and the dangers of the more energetic waves.
A focused answer to AQA GCSE Physics 4.6.2, covering the seven groups of the electromagnetic spectrum in order, their shared properties as transverse waves, their main uses, and the dangers of ultraviolet, X-rays and gamma rays.
- Reflection and refraction: how waves are reflected, transmitted or absorbed at a boundary, the law of reflection, and why refraction occurs (separate physics).
A focused answer to AQA GCSE Physics 4.6.1 and 4.6.2, covering how waves are reflected, transmitted or absorbed at a boundary, the law of reflection with specular and diffuse reflection, and why refraction occurs when a wave changes speed.
- Sound and uses of waves: how sound travels through solids and is heard, the range of human hearing, ultrasound, and the use of waves in detection and imaging (separate physics).
A focused answer to AQA GCSE Physics 4.6.1 and 4.6.2, covering how sound travels as a longitudinal wave and is heard, the human hearing range, ultrasound and its uses, and how reflected waves are used for detection and imaging such as echo sounding and seismic waves.
- Lenses and visible light: how convex and concave lenses refract light, ray diagrams and magnification, and how colour depends on reflection, transmission and absorption (separate physics).
A focused answer to AQA GCSE Physics 4.6.2, covering how convex and concave lenses refract light to form images, drawing ray diagrams and using the magnification equation, and how the colour of objects depends on the reflection, transmission and absorption of light.
Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)