Waves, surf and otherwise, are vibrations that transfer energy through a medium (solid, liquid or gas) or a vacuum without matter being transferred. Sound and seismic waves travel through a medium (air, the Earth’s core); electromagnetic waves (radio, microwaves, visible light) also travel through empty space.
All electromagnetic waves and water waves and some seismic waves (S waves) are transverse, vibrating at right angles to their direction of travel. Sound waves and some seismic waves (P waves) are longitudinal. These vibrate in the same direction as their direction of travel.
Frequency (Hertz), amplitude (the size of disturbance caused) and wavelength (metres) determine wave type.
Earthquakes: Rockers and a hard place
The vibrations triggered by earthquakes are seismic waves, either the compressional type (P waves) or the up-and-down type (S waves).
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Seismic waves spread through the Earth’s interior but seismic velocities depend on the medium’s composition, mineral phase and packing structure, temperature and pressure. Seismic waves travel more quickly through denser materials and so generally travel more quickly with depth. Hot areas slow down seismic waves, which also move more slowly through a liquid than a solid.
Almost everything we know about the interior of the Earth has come from measuring seismic waves. Reconstructing the paths of seismic ripples tells us that the very centre of the Earth is probably a solid iron core.
Radar: Bouncing back
Radar (Rapid Automatic Detection and Alignment of Repeats) uses radio waves to detect the presence of objects at a distance. Radio waves travel at a constant speed in air, so by sending out a pulse and measuring the time it takes to return it is possible to calculate the distance of an object and map out the profile of its surface.
By sending a continuous stream of pulses, radar can also detect the trajectory of the object — the pulses take longer and longer to return as an object fades into the distance, for instance. Radar is used in military aircraft, air traffic control, shipping, to map the surface of the Earth, by police forces to detect speeding vehicles and as a sensing system by automatic doors in shops.
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Digital communication: The light fantastic
Modern communications are seemingly instantaneous, but a physical journey is still required to send data. Digital information is transmitted as light waves along fibre-optic lines, which are strands of optically pure glass as thin as a human hair. The inner fibre is wrapped in a cladding that acts as an insulator, causing light to bounce back and forth internally and transmit the light almost without loss.
Modern fibres operate several “channels” simultaneously by carrying light of different wavelengths, which can be separated out with the modern equivalent of a prism at the end of the line. So little light escapes that it is now possible to send 14 trillion bits, or 14 Terabits, per second over 160 kilometres of fibre.
Ultrasound: Pulsating images
Ultrasound waves operate at a frequency higher than the limit of human hearing (we can hear sounds with frequencies between 20 and 20,000Hz — any higher is ultrasound). They are used, most miraculously, to produce pictures of the foetus inside the human womb, but can also “see” into other parts of the body. The longitudinal waves are projected, via a friction-reducing gel, through the skin, bouncing back when they strike an object — a tiny head, wrist or toe. This allows computer software to construct an image based on the timings between the reverberations.
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Ultrasound can also be used for industrial cleaning — high-powered waves accelerate particle disintegration, killing bacteria in sewage or quickening putrefaction in waste.
Cooking: Gourmet waves
Long before that metal box in the kitchen appropriated the name of the energy it employed, microwaves were used in telecommunications, radar and navigation. But it is for cooking that they’re best known.
Sandwiched between radio and infrared waves on the electromagnetic spectrum, and with wavelengths from 1mm to 50cm, they penetrate about 1cm into the food, causing its water molecules to vibrate, which in turn cause neighbouring molecules of food to vibrate. This further energises the molecules, heating the water and cooking the food.
Why no metal? Powerful electromagnetic waves can dislodge electrons from metals, creating an electrical charge in the air — an event that manifests as a spark.