"Wave phenomenon" is a phrase that appears in physics syllabuses from Grade 9 to the first year of university, and it means something exact: a behaviour that only happens because something is travelling as a wave. There are seven that school and college physics cares about, each has an everyday example you have already seen, and each has a standard exam question. This page gives all of that, and the few equations that go with them.
What a wave is
A wave is a disturbance that carries energy from one place to another without carrying the material it travels through. The cork on the pond bobs up and down as the ripple passes and stays where it was. The air in a room vibrates back and forth as sound crosses it and does not move to the other side. That is the definition to learn, because the exam question "explain how a wave transfers energy without transferring matter" is asked at every level.
Waves come in two kinds. In a transverse wave the vibration is at right angles to the direction of travel: waves on a string, water waves at the surface, light and all electromagnetic waves. In a longitudinal wave the vibration is along the direction of travel: sound, and the compressions along a slinky. The distinction matters for one of the seven phenomena below, polarisation, which only transverse waves show.
The quantities every wave has: wavelength λ, the distance between one crest and the next; frequency f, how many waves pass a point each second; period T = 1/f; amplitude, the maximum displacement from rest; and speed v = f λ. That last equation is the one used in nearly every wave calculation before university.
The seven wave phenomena
| Phenomenon | What happens | Everyday example | The rule or equation |
|---|---|---|---|
| Reflection | The wave bounces off a surface | A mirror; an echo; the sea wall | Angle of incidence equals angle of reflection |
| Refraction | The wave changes direction when its speed changes, usually on entering a new medium | A straw looking bent in water; a lens; a swimming pool looking shallower than it is | Snell's law: n₁ sin θ₁ = n₂ sin θ₂; the wave slows in the denser medium and bends toward the normal |
| Diffraction | The wave spreads out after passing an edge or a gap | Hearing someone around a corner; water waves spreading through a harbour entrance; the fuzzy edge of a shadow | Greatest when the gap is about the same size as the wavelength |
| Interference | Two waves meet and add (constructive) or cancel (destructive) | Colours in a soap bubble or oil film; noise-cancelling headphones; Young's double slit | Constructive where the path difference is a whole number of wavelengths; destructive at half-integers |
| Polarisation | The vibration is restricted to one plane | Polarised sunglasses cutting glare from water; 3D cinema glasses; LCD screens | Only transverse waves can be polarised, which is how we know light is transverse |
| The Doppler effect | The observed frequency changes when the source or observer moves | A siren's pitch dropping as the ambulance passes; the redshift of galaxies; radar speed guns | Approaching: frequency rises, wavelength shortens; receding: the reverse |
| Standing waves | A wave and its reflection combine to make a fixed pattern of nodes and antinodes | A guitar string; the air in a flute or organ pipe; a microwave oven's hot and cold spots | Resonant lengths: a string fixed at both ends fits whole numbers of half-wavelengths |
Each one in a little more depth
Reflection is the simplest and the first taught. The law, angle in equals angle out, is measured from the normal, the line at right angles to the surface; most lost marks at Grade 9 and 10 come from measuring from the surface instead. Echoes are reflection of sound; the delay gives the distance (sound travels about 340 metres a second in air, so a one-second echo means a wall 170 metres away).
Refraction happens because the wave changes speed. Light slows in glass or water, and a wavefront that hits the surface at an angle has one end slowed before the other, so it swings round: toward the normal going in, away from it coming out. Every lens, every pair of glasses and every rainbow is refraction; the rainbow adds dispersion, which is refraction by an amount that depends on the colour, so white light fans into the spectrum.
Diffraction is the first behaviour that particles cannot do. A stream of particles through a doorway goes straight; a wave spreads out. The amount of spreading depends on the ratio of the gap to the wavelength: a doorway is about the size of a sound wavelength, so sound spreads round it well and you hear the conversation in the next room, while light's wavelength is a million times smaller, so light through the same doorway barely spreads and you cannot see round the corner.
Interference is the second behaviour that particles cannot do, and the one that settled the argument about light. When two waves meet, their displacements add. Where crest meets crest the result is larger (constructive); where crest meets trough they cancel (destructive). Young's double-slit experiment in 1801 sent light through two narrow slits and produced a pattern of bright and dark fringes on a screen, which only interference can explain. The fringe spacing x for slits a apart and a screen D away gives the wavelength: λ = a x / D, an A-level and IB favourite. The colours in a soap bubble are interference between light reflected from the front and back of the film; noise-cancelling headphones generate a sound wave that is the inverse of the noise, so the two cancel.
Polarisation only works for transverse waves, because it means restricting the vibration to one plane, and a longitudinal wave has no plane to restrict. Light from the sun vibrates in every plane; light reflected off water or a road is mostly polarised horizontally; polarised sunglasses have a filter that passes only vertical vibration, so the glare is cut and the rest is not. That light can be polarised is the proof that it is a transverse wave.
The Doppler effect is a change in observed frequency when the source and observer move relative to each other. The ambulance siren is the standard example: the waves ahead of the moving ambulance are bunched together (higher frequency, higher pitch), the ones behind are stretched (lower). The same effect on light gives redshift: light from galaxies moving away is stretched toward the red end of the spectrum, which is how we know the universe is expanding. Radar speed guns and medical ultrasound of blood flow both use it.
Standing waves form when a wave reflects back on itself in a confined space and the two interfere to give a pattern that does not travel: points of no movement (nodes) and maximum movement (antinodes). A guitar string fixed at both ends can only hold patterns that fit whole numbers of half-wavelengths, which is why it plays particular notes; a wind instrument does the same with air in a pipe. Standing-wave questions on strings and pipes are set in every A-level, IB and AP Physics 1 paper.
Why these phenomena matter: wave or particle?
For two centuries physicists argued whether light was a stream of particles or a wave. Reflection and refraction could be explained either way. Diffraction and interference could not: two particles cannot cancel each other, two waves can. Young's fringes settled it for a hundred years, until the photoelectric effect showed light also behaves as particles, which is where quantum physics begins. A student who can say which phenomena are wave-only (diffraction, interference, polarisation) and which are not (reflection, refraction) has the answer to one of the commonest six-mark questions in the subject.
The exam questions on each
| Level | What is set |
|---|---|
| IGCSE, GCSE, CBSE Class 9 and 10 | Draw and label a transverse and a longitudinal wave; use v = f λ; the law of reflection with a ray diagram; refraction toward or away from the normal; describe diffraction through a gap; the echo calculation |
| A-level, IB, AP Physics 1 and 2 | Snell's law with refractive index; total internal reflection and the critical angle; Young's double slit and λ = a x / D; diffraction gratings; polarisation and Malus's law; the Doppler effect qualitatively and in astronomy; standing waves on strings and in pipes with harmonics |
| CBSE Class 11 and 12 | Wave motion and the equation of a progressive wave; superposition and standing waves in Class 11; wave optics in Class 12: Huygens' principle, interference, Young's experiment, diffraction at a single slit, polarisation |
The marks are lost in the same places at every level: measuring angles from the surface instead of the normal; confusing diffraction (one gap, spreading) with interference (two or more sources, fringes); saying a wave "bends" in refraction without saying it changes speed; and in the Doppler effect, saying the pitch changes because the sound gets louder. Each is a sentence to learn, and the method for learning physics as sentences is in how to study physics.
Where to get help
A 1-on-1 online physics class with a tutor who holds a master's in physics: waves taught with diagrams on a shared whiteboard, the seven phenomena as sentences the student can say, and the exam board's own questions on each marked to the scheme. For IGCSE, GCSE, A-level, IB, AP Physics 1 and 2, and CBSE Class 11 and 12. The free 30-minute first class is a real lesson on the topic you bring.
Questions parents ask
1What is a wave phenomenon?
2What are examples of wave phenomena?
3What are the wave phenomena of light?
4What is the difference between a wave and a particle?
5What is the wave equation in physics?
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