phase-contrast microscope: see microscope.

phase modulation: see modulation.

phase diagram, graph that shows the relation between the solid, liquid, and gaseous states of a substance (see states of matter) as a function of the temperature and pressure. The graph is divided into three regions, one for each of the physical states, and it specifies the range of temperatures at which the substance exists in each state for any value of the pressure. For example, a phase diagram for water shows that at a pressure of 1 atmosphere water is a solid up to a temperature of 0°C;, a liquid from 0°C; to 100°C;, and a gas above 100°C;. At a pressure of 0.5 atmospheres, the graph shows that although the melting point of ice remains 0°C;, the boiling point of water is lowered to 82°C;. Each substance has its own phase diagram, which must be determined experimentally. The border between two regions on the graph represents an equilibrium state, such as a melting point or boiling point, at which two states can coexist (see chemical equilibrium). The point at which all three regions meet is called the triple point; at these conditions of temperature and pressure, the solid, liquid, and gaseous states can coexist in equilibrium. The triple-point conditions for water are a temperature of .01°C; and a pressure of .006 atmospheres.

phase, in astronomy, the measure of how much of the illuminated surface of a planet or satellite can be seen from a point at a distance from that body; the term is most often used to describe the moon as seen from the earth. When the moon is between the earth and the sun, we cannot see the lighted half at all, and the moon is said to be new. For a few days before and after the new moon we can see a small part of the lighted half, which appears as a crescent with the horns, or cusps, pointing away from the sun. When the moon has completed half its orbit from new moon to new moon, it is on the opposite side of the earth from the sun and we see the entire lighted half; this phase is called the full moon. When the moon is at quadrature with the sun, having completed either one quarter or three quarters of its orbit from new moon to new moon, half the lighted side is visible; this phase is called the half-moon. The half-moon between the new moon and the full moon is known as the first quarter and that between the full moon and new moon is known as the last quarter. Between the first quarter and the full moon and between the full moon and the last quarter we see more than half the lighted side; this phase is called gibbous. Of the planets, only Mercury and Venus, whose orbits pass between the earth and sun, show all the phases that the moon shows; the other planets are always either gibbous or full.

phase, in physics: see wave.

gibbous phase: see phase, in astronomy.

In wave motion, the fraction of the time required to complete a full cycle that a point completes after last passing through the reference position. Two periodic motions are said to be in phase when corresponding points of each reach maximum or minimum displacements at the same time. If the crests of two waves pass the same point at the same time, they are in phase for that position. If the crest of one and the trough of the other pass the same point at the same time, the phase angles differ by 180° and the waves are said to be of opposite phase. Phase differences are important in alternating electric current technology (see alternating current).

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