A wave is a disturbance that transfers energy from one place to another without transferring matter. Waves explain light, sound, radio, earthquakes and ripples on water. CSEC Physics asks you to describe the types and properties of waves, use the wave equation, and know the electromagnetic spectrum.
Two types of wave
- Transverse waves: the particles (or fields) vibrate at right angles to the direction the wave travels. Examples: light and all electromagnetic waves, water ripples, waves on a rope. They have crests and troughs.
- Longitudinal waves: the particles vibrate parallel to the direction of travel, producing compressions (particles squeezed together) and rarefactions (spread apart). The main example is sound.
In both cases energy moves along but the particles only vibrate about fixed positions โ they are not carried with the wave.
Describing a wave
- Amplitude โ the maximum displacement from the rest position. Larger amplitude means more energy (a louder sound or brighter light).
- Wavelength (ฮป) โ the distance for one complete wave (e.g. crest to crest), measured in metres.
- Frequency (f) โ the number of complete waves passing a point per second, measured in hertz (Hz).
- Period (T) โ the time for one complete wave; T = 1/f.
- Wave speed (v) โ how fast the wave travels.
The wave equation
These are linked by:
wave speed = frequency ร wavelength v = f ร ฮป
For example, a wave of frequency 50 Hz and wavelength 6 m travels at 50 ร 6 = 300 m/s. Rearrange to find any quantity given the other two.
Wave behaviour
- Reflection: a wave bounces off a barrier; the angle of incidence equals the angle of reflection. (Echoes are reflected sound; mirrors reflect light.)
- Refraction: a wave changes speed, and usually direction, when it passes into a different medium (e.g. light bending as it enters glass or water).
- Diffraction: waves spread out when they pass through a gap or around an obstacle; the effect is greatest when the gap is about the same size as the wavelength.
The electromagnetic spectrum
Electromagnetic (EM) waves are transverse, can travel through a vacuum, and all travel at the same speed in a vacuum (the speed of light, 3 ร 10โธ m/s). They differ in wavelength and frequency. In order of increasing frequency (decreasing wavelength):
radio โ microwave โ infrared โ visible light โ ultraviolet โ X-rays โ gamma rays
Each has uses and, at the high-frequency end, dangers:
- Radio: communication, broadcasting.
- Microwave: cooking, mobile phones, satellite links.
- Infrared: heating, remote controls, thermal imaging.
- Visible light: sight, photography, optical fibres.
- Ultraviolet: sterilising, security marking โ but causes sunburn and skin cancer.
- X-rays: medical imaging of bones โ but ionising, can damage cells.
- Gamma rays: sterilising equipment, treating cancer โ but highly ionising and dangerous.
The higher the frequency, the more energy the wave carries and the more harmful it is to living tissue.
Worked examples with the wave equation
The relationship v = f ฮป is one of the most-tested in the paper, so practise rearranging it:
- Finding speed: a wave has frequency 25 Hz and wavelength 0.2 m. Speed = f ร ฮป = 25 ร 0.2 = 5 m/s.
- Finding wavelength: a radio station broadcasts at 200,000 Hz (200 kHz); radio waves travel at 3 ร 10โธ m/s. Wavelength = v รท f = 300,000,000 รท 200,000 = 1500 m.
- Finding frequency: a sound wave travels at 340 m/s with a wavelength of 0.85 m. Frequency = v รท ฮป = 340 รท 0.85 = 400 Hz.
Always convert prefixes (kHz to Hz, cm to m) before substituting, and quote the unit in the answer.
The ripple tank
Many wave properties are demonstrated using a ripple tank โ a shallow tray of water with a vibrating bar that makes straight or circular waves, lit from above so the wave pattern shows on a screen below. With it you can see:
- reflection off a barrier (the waves bounce back at an equal angle);
- refraction as waves slow down and change direction over a shallower region (a glass plate placed in the tank);
- diffraction as waves spread out after passing through a narrow gap.
Because water waves are easy to see, the ripple tank is the standard way CSEC introduces wave behaviour, and questions often ask you to describe what is observed and why. The same behaviours โ reflection, refraction and diffraction โ apply to light and sound, even though those are harder to watch directly.
Common exam mistakes
- Saying waves transfer matter โ they transfer energy, not matter.
- Confusing transverse (vibration at right angles) with longitudinal (vibration parallel).
- Getting the spectrum order wrong โ learn it from radio (longest wavelength) to gamma (shortest).
- Forgetting units, or not converting (e.g. kHz to Hz) before using v = fฮป.
Light, reflection and refraction
Because light is a wave, it shows the wave behaviours in everyday ways that CSEC asks about:
- Reflection in a plane mirror gives an image that is the same size, upright, and as far behind the mirror as the object is in front (a virtual image).
- Refraction is the bending of light as it changes speed entering glass or water; it explains why a straw looks bent in a glass of water and why a swimming pool looks shallower than it is.
- When light slows entering a denser medium it bends towards the normal; speeding up on leaving, it bends away.
- At a large enough angle inside glass or water, light is completely reflected back โ total internal reflection โ which is how optical fibres carry light (and telephone and internet signals) along their length.
These optical effects are just the general wave properties of reflection and refraction applied to visible light.
Key terms to remember
- Wave โ a disturbance that transfers energy without transferring matter.
- Transverse wave โ vibration at right angles to travel (e.g. light); has crests and troughs.
- Longitudinal wave โ vibration parallel to travel (e.g. sound); has compressions and rarefactions.
- Amplitude โ maximum displacement from rest; relates to energy.
- Wavelength (ฮป) โ distance for one complete wave.
- Frequency (f) โ waves per second, in hertz (Hz); period T = 1/f.
- Wave equation โ v = f ฮป.
- Electromagnetic spectrum โ radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Quick recap
- A wave transfers energy without matter; transverse vibrate at right angles, longitudinal vibrate parallel (sound).
- Key terms: amplitude (energy), wavelength, frequency (Hz), period (T = 1/f).
- v = f ฮป links speed, frequency and wavelength.
- Waves can reflect, refract and diffract.
- The EM spectrum (radio โ gamma) is transverse, travels at the speed of light in a vacuum; higher frequency = more energy and more hazard.