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Waves traveling through a solid medium can be either transverse waves or longitudinal waves. Yet waves traveling through the bulk of a fluid (such as a liquid or a gas) are always longitudinal waves. Transverse waves require a relatively rigid medium in order to transmit their energy. As one particle begins to move it must be able to exert a pull on its nearest neighbor. If the medium is not rigid as is the case with fluids, the particles will slide past each other. This sliding action which is characteristic of liquids and gases prevents one particle from displacing its neighbor in a direction perpendicular to the energy transport. It is for this reason that only longitudinal waves are observed moving through the bulk of liquids such as our oceans. Earthquakes are capable of producing both transverse and longitudinal waves which travel through the solid structures of the Earth. When seismologists began to study earthquake waves they noticed that only longitudinal waves were capable of traveling through the core of the Earth. For this reason, geologists believe that the Earth's core consists of a liquid - most likely molten iron.
Of two rays of light match each other perfectly in color, they can interact in a surprising way. Because all the crests of one wave have the same wavelength as the second ray the crests of the two waves can be lined up with each other. As each wave crest of one ray coincides with the crest of the other ray, the two amplitudes of the waves add up to twice the amplitude and the result is a single, much brighter light ray. This is called constructive interference. (Probably the only time when it is considered constructive to interfere!)
Interferometry is the science and technique of superposing (interfering) two or more waves, which creates an output wave different from the input waves; this in turn can be used to explore the differences between the input waves. Because interference is a very general phenomenon with waves, interferometry can be applied to a wide variety of fields, including astronomy, fiber optics, optical metrology, oceanography, seismology and various studies of quantum mechanics. Interferometry can be applied to both one-dimensional waves such as time varying signals, or to multi-dimensional waves such as coherent images produced by laser illumination.