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In physics, oscillation that is propagated from a source. Mechanical waves require a medium through which to travel. Electromagnetic waves do not; they can travel through a vacuum. Waves carry energy but they do not transfer matter. The medium (for example the Earth, for seismic waves) is not permanently displaced by the passage of a wave. The model of waves as a pattern is used to help understand the properties of light and sound. Experiments conducted in a ripple tank with water waves can explain how waves slow down as water becomes shallower, how waves change direction when travelling through another medium, and how waves are reflected from different surfaces. See also standing wave.
Types of wave
There are various ways of classifying wave types. One of these is based on the way the wave travels. In a transverse wave, the displacement of the medium is perpendicular to the direction in which the wave travels. An example of this type of wave is a mechanical wave projected along a tight string. The string moves at right angles to the wave motion. Electromagnetic waves are another example of transverse waves. The directions of the electric and magnetic fields are perpendicular to the wave motion. In a longitudinal wave the disturbance takes place parallel to the wave motion. A longitudinal wave consists of a series of compressions and rarefactions (states of maximum and minimum density and pressure, respectively). Such waves are always mechanical in nature and thus require a medium through which to travel. Sound waves are an example of longitudinal waves. Waves that result from a stone being dropped into water appear as a series of circles. These are called circular waves and can be generated in a ripple tank for study. Waves on water that appear as a series of parallel lines are called plane waves.Characteristics of waves
All waves have a wavelength. This is measured as the distance between successive crests (or successive troughs) of the wave. It is given the Greek symbol λ. The frequency of a wave is the number of vibrations per second. It is expressed in hertz, symbol Hz (1 Hz = 1 cycle per second). The reciprocal of this is the wave period. This is the time taken for one complete cycle of the wave oscillation. The speed of the wave is measured by multiplying wave frequency by the wavelength.Properties of waves
When a wave moves from one medium to another (for example a light wave moving from air to glass) it moves with a different speed in the second medium. This change in speed causes it to change direction. This property is called refraction. The angle of refraction depends on whether the wave is speeding up or slowing down as it changes medium. Reflection occurs whenever a wave hits a barrier. The wave is sent back, or reflected, into the medium. The angle of incidence (the angle between the ray and a perpendicular line drawn to the surface) is equal to the angle of reflection (the angle between the reflected ray and a perpendicular to the surface). See also total internal reflection. An echo is the repetition of a sound wave by reflection from a surface. All waves spread slightly as they travel. This is called diffraction and it occurs chiefly when a wave interacts with a solid object. The degree of diffraction depends on the relationship between the wavelength and the size of the object (or gap through which the wave travels). If the two are similar in size, diffraction occurs and the wave can be seen to spread out. Large objects cast shadows because the difference between their size and the wavelength is so large that light waves are not diffracted around the object. A dark shadow results. When two or more waves meet at a point, they interact and combine to produce a resultant wave of larger or smaller amplitude (depending on whether the combining waves are in or out of phase with each other). This is called interference. Transverse waves can exhibit polarization. If the oscillations of the wave take place in many different directions (all at right angles to the directions of the wave) the wave is unpolarized. If the oscillations occur in one plane only, the wave is polarized. Light, which consists of transverse waves, can be polarized.The amplitude of oscillationsThe amplitude is the maximum displacement of an oscillation from the equilibrium position (the height of a crest or the depth of a trough). With a sound wave, for example, amplitude corresponds to the intensity (loudness) of the sound. If a mechanical system is made to vibrate by applying oscillations to it, the system vibrates. As the frequency of the oscillations is varied, the amplitude of the vibrations reaches a maximum at the natural frequency of the system. If a force with a frequency equal to the natural frequency is applied, the vibrations can become violent, a phenomenon known as resonance. |
Electromagnetic wavesAll electromagnetic waves travel at the same speed, the speed of light. They vary in wavelength and frequency to give the broad range of waves found in the electromagnetic spectrum. They include radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Many of the colours observed in everyday objects are seen because of the phenomenon of the absorption of light. Rainbows display the colours of the visible spectrum and are formed by the refraction and reflection of the Sun's rays through rain or mist. |
Optical instrumentsLight can be detected by the eye and by cameras. A simple device for producing an image is the pinhole camera. Light passes through the pinhole at one end of a box to form a sharp inverted image on the inside surface of the opposite end. The image is equally sharp for objects placed at different distances from the camera. In the eye, a converging lens focuses light from a distant object on the retina at the back of the eye. In normal vision, the image is focused to a fine point and light-sensitive cells on the retina send signals to the brain where the image is interpreted. In short-sightedness, the rays of light from a distant object are focused in front of the retina so that the image appears blurred. This can be corrected by using a concave lens (or diverging lens) in front of the eye. In long-sightedness, the rays of light from a near object are focused behind the retina. This can be corrected by using a convex lens (or converging lens) in front of the eye. A concave mirror converges light rays to form a reduced, inverted, real image in front, or an enlarged, upright, virtual image seemingly behind it, depending on how close the object is to the mirror. (A real image is one through which the light rays pass; a virtual image is one through which they do not pass.) A formula can be used to calculate the image position if the object position and focal length values are known. If parallel rays are directed at a convex mirror, the reflected rays diverge to form a reduced, upright, virtual image. The transmission of light and images through glass or plastic fibres, known as optical fibres, is called fibre optics. |
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In the oceans, a ridge or swell formed by wind or other causes. The power of a wave is determined by the strength of the wind and the distance of open water over which the wind blows (the fetch). Waves are the main agents of coastal erosion and deposition: sweeping away or building up beaches, creating spits and berms, and wearing down cliffs by their hydraulic action and by the corrosion of the sand and shingle that they carry. A tsunami (misleadingly called a ‘tidal wave’) is formed after a submarine earthquake.
As a wave approaches the shore it is forced to break as a result of friction with the seabed. When it breaks on a beach, water and sediment are carried up the beach as swash; the water then drains back as backwash.
A constructive wave causes a net deposition of material on the shore because its swash is stronger than its backwash. Such waves tend be low and have crests that spill over gradually as they break. The backwash of a destructive wave is stronger than its swash, and therefore causes a net removal of material from the shore. Destructive waves are usually tall and have peaked crests that plunge downwards as they break, trapping air as they do so.
If waves strike a beach at an angle the beach material is gradually moved along the shore (longshore drift), causing deposition of material in some areas and erosion in others.
Atmospheric instability caused by the greenhouse effect and global warming appears to be increasing the severity of Atlantic storms and the heights of the ocean waves. Waves in the South Atlantic are shrinking – they are on average half a metre smaller than in the mid-1980s – and those in the northeast Atlantic have doubled in size over the last 40 years. As the height of waves affects the supply of marine food, this could affect fish stocks, and there are also implications for shipping and oil and gas rigs in the North Atlantic, which will need to be strengthened if they are to avoid damage.
Freak or ‘episodic’ waves form under particular weather conditions at certain times of the year, travelling long distances across the Atlantic, Indian, and Pacific oceans. They are considered responsible for the sudden unexplained disappearance of many ships.
Freak waves become extremely dangerous when they reach the shallow waters of the continental shelves at 100 fathoms (180 m/600 ft), especially when they meet currents: for example, the Agulhas Current to the east of South Africa, and the Gulf Stream in the North Atlantic. A wave height of 34 m/112 ft has been recorded.
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