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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !
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Tips: browse the semantic fields (see From ideas to words) in two languages to learn more.
Surface or ground waves can be of mechanical or electromagnetic nature. In physics, a surface wave is a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave guided by a refractive index gradient. In radio transmission, a ground wave is a surface wave that propagates close to the surface of the Earth.
In seismology, several types of surface waves are encountered. Surface waves, in this mechanical sense, are commonly known as either Love waves (L waves) or Rayleigh waves. A seismic wave is a wave that travels through the Earth, often as the result of an earthquake or explosion. Love waves have transverse motion (movement is perpendicular to the direction of travel, like light waves), whereas Rayleigh waves have both longitudinal (movement parallel to the direction of travel, like sound waves) and transverse motion. Seismic waves are studied by seismologists and measured by a seismograph or seismometer. Surface waves span a wide frequency range, and the period of waves that are most damaging is usually 10 seconds or longer. Surface waves can travel around the globe many times from the largest earthquakes.
The term "surface wave" can describe waves over an ocean, even when they are approximated by Airy functions and are more properly called creeping waves. Examples are the waves at the surface of water and air (ocean surface waves), or ripples in the sand at the interface with water or air. Another example is internal waves, which can be transmitted along the interface of two water masses of different densities.
Ground waves refer to the propagation of radio waves parallel to and adjacent to the surface of the Earth, following the curvature of the Earth. These surface waves are also known loosely as Norton surface waves, Zenneck waves, Sommerfeld waves, or gliding waves.
Lower frequencies, below 3 MHz, travel efficiently as ground waves. This is because they are more strongly diffracted around obstacles due to their long wavelengths, allowing them to follow the Earth's curvature. Ionospheric reflection is taken into consideration as well. The ionosphere reflects frequencies in a certain band, which often changes due to solar conditions. The Earth has one refractive index and the atmosphere has another, thus constituting an interface that supports the surface wave transmission. Ground waves propagate in vertical polarization, with their magnetic field horizontal and electric field (close to) vertical.
Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as water providing better propagation. Increasing the conductivity in a surface results in less dissipation. The refractive indices are subject to spatial and temporal changes. Since the ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth’s surface. The wavefronts initially are vertical, but the Earth's greater refractive index causes the wavefronts to "lean over" at a progressively greater angle in the direction of propagation as they travel, until they are dissipated.
Most long-distance LF "longwave" radio communication (between 30 kHz and 300 kHz) is a result of groundwave propagation. Mediumwave radio transmissions (frequencies between 300 kHz and 3000 kHz), including AM broadcast band, travel both as groundwaves and, for longer distances, as skywaves. Ground losses become lower at lower frequencies, so the attenuation of ground waves decreases with frequency. The VLF and LF frequencies are mostly used for military communications, especially with ships and submarines.
Surface waves have been used in over-the-horizon radar, which operates mainly at frequencies between 2 and 20 MHz over the sea, which has a sufficiently high conductivity to convey the surface waves to and from a reasonable distance (up to 100 km or more; over-horizon radar also uses skywave propagation at much greater distances). In the development of radio, surface waves were used extensively. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the higher (HF) frequencies, felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined only to authorized frequencies in the range.
Mediumwave and shortwave reflect off the ionosphere at night, which is known as skywave. During daylight hours, the lower "D" layer of the ionosphere forms and absorbs lower frequency energy. This prevents skywave propagation from being very effective on mediumwave frequencies in daylight hours. At night, when the "D" layer dissipates, mediumwave transmissions travel better by skywave. Ground waves do not include ionospheric and tropospheric waves.
Within microwave field theory, the interface of a dielectric and conductor supports "surface wave transmission." Surface waves have been studied as part of transmission lines and some may be considered as single-wire transmission lines.
Characteristics and utilizations of the electrical surface wave phenomena include: