An important factor in AC circuit operation is the time at which the instantaneous current or voltage goes through some specified point in the waveform. This is called the phase of the wave.If there are two waves the phase difference between the two can be expressed either as a time difference or, if the waveform is periodic, as the appropriate fraction of either the period or cycle. The cycle is generally the preferred reference.

phage = P = phase of the moon

phase

1. n. The offset of one's waking-sleeping schedule with respect to the standard 24-hour cycle; a useful concept among people who often work at night and/or according to no fixed schedule. It is not uncommon to change one's phase by as much as 6 hours per day on a regular basis. "What's your phase?" "I've been getting in about 8 P.M. lately, but I'm going to wrap around to the day schedule by Friday." A person who is roughly 12 hours out of phase is sometimes said to be in `night mode'. (The term `day mode' is also (but less frequently) used, meaning you're working 9 to 5 (or, more likely, 10 to 6).) The act of altering one's cycle is called `changing phase'; `phase shifting' has also been recently reported from Caltech. 2. `change phase the hard way': To stay awake for a very long time in order to get into a different phase. 3. `change phase the easy way': To stay asleep, etc. However, some claim that either staying awake longer or sleeping longer is easy, and that it is shortening your day or night that is really hard (see wrap around). The `jet lag' that afflicts travelers who cross many time-zone boundaries may be attributed to two distinct causes: the strain of travel per se, and the strain of changing phase. Hackers who suddenly find that they must change phase drastically in a short period of time, particularly the hard way, experience something very like jet lag without traveling.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Optics: Offset of the electric or magnetic field from some predefined "zero" in propagating light. Since both fields are cyclical in nature, the phase will return to zero at the end of each cycle. The "absolute phase" is rarely of interest, in most situations only the "relative phase" is important. (it is also impossible to determine the "absolute phase in most cases anyway.)

For instance many optics situations assume a plane wave which is a optical wavefront or line of constant phase as the starting point. After going through some system (a lens for instance), the wavefront is no longer straight (in the case of the lens it's bowed in the shape of the physical lens because the light is delayed proportional to the time spent in the lens.) The shape of the wavefront tells us how the system will change light going through it.

An explanation of almost all optical devices or systems requires an intuitive understanding of optical phase and it's consequences.

Phase (?), n.; pl. Phases (#). [NL. phasis, Gr. &?;, fr. &?; to make to appear: cf. F. phase. See Phenomenon, Phantom, and Emphasis.]

1.

That which is exhibited to the eye; the appearance which anything manifests, especially any one among different and varying appearances of the same object.

2.

Any appearance or aspect of an object of mental apprehension or view; as, the problem has many phases.

3. (Astron.)

A particular appearance or state in a regularly recurring cycle of changes with respect to quantity of illumination or form of enlightened disk; as, the phases of the moon or planets. See Illust. under Moon.

4. (Physics)

Any one point or portion in a recurring series of changes, as in the changes of motion of one of the particles constituting a wave or vibration; one portion of a series of such changes, in distinction from a contrasted portion, as the portion on one side of a position of equilibrium, in contrast with that on the opposite side.

Phase (?), n.

1. (Phys. Chem.)

A homogenous, physically distinct portion of matter in a system not homogeneous; as, the three phases, ice, water, and aqueous vapor. A phase may be either a single chemical substance or a mixture, as of gases.

2. (Zoöl.)

In certain birds and mammals, one of two or more color variations characteristic of the species, but independent of the ordinary seasonal and sexual differences, and often also of age. Some of the herons which appear in white and colored phases, and certain squirrels which are sometimes uniformly blackish instead of the usual coloration, furnish examples. Color phases occur also in other animals, notably in butterflies.

3. (Elec.)

The relation at any instant of a periodically varying electric magnitude, as electro-motive force, a current, etc., to its initial value as expressed in factorial parts of the complete cycle. It is usually expressed in angular measure, the cycle beb four right angles, or 360°. Such periodic variations are generally well represented by sine curves; and phase relations are shown by the relative positions of the crests and hollows of such curves. Magnitudes which have the same phase are said to be in phase.

Phase (?), v. t. [Cf. Feeze.]

To disturb the composure of; to disconcert; to nonplus. [Colloq.]

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