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(for example, selenium and cadmium oxides), light energy is absorbed into the material, and electrons are
forced to move through it. This movement of electrons is photoelectricity.
photoelectric material. A material that emits electrons when light shines on it. Some of the alkaline metals,
such as cesium, lithium, and rubidium, are photoelectric.
photoemissivity. The characteristic of a material that causes it to emit electrons when it is exposed to light.
photofet. A junction field effect transistor in which the gate-to-channel junction acts as a photodiode. The
light sensitivity of a photofet is about ten times as great as that of an ordinary phototransistor.
photon. A massless particle of an electromagnetic field. A photon, which is also called a quantum of light,
has both energy and momentum.
photonegative characteristics. The characteristics of a material that cause its resistance to increase when
light shines on it.
photophone. A device that changes variations of light intensity into sound. A beam of light can be
modulated by the output of a microphone. If this modulated light beam is shone on a photoelectric cell, its
output current will duplicate the current produced by the microphone.
photo reconnaissance (air traffic control). Military activity that requires locating individual photo targets
and navigating to the targets at a preplanned angle and altitude. Photo reconnaissance normally requires a
lateral route width of 16 nautical miles and an altitude range of 1,500 feet to 10,000 feet AGL.
photoresistor. A semiconductor device that acts as a variable resistor whose resistance is determined by
the amount of light shining on it through a built-in lens. The resistance of a photoresistor increases as the
amount of light shining on it increases.
photothyristor. A light-activated silicon controlled rectifier (LASCR). Light falling on the gate-cathode
junction triggers the LASCR into conduction.
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phototransistor. A junction transistor in which emitter-collector current flows when light shines on the
reverse-biased collector-base junction. A small window in the housing allows light to shine on the junction
and control the current. Phototransistors produce a current gain and are often used in place of photodiodes.
photovoltaic cell. A solid-state electrical component that produces a voltage when it is exposed to light.
phugoid oscillation. Oscillations of an aircraft about its pitch, or lateral, axis (along its longitudinal axis).
These oscillations consist of long-term, low-amplitude climbs and dives, with little or no change in the
angle of attack.
physics. The branch of natural science that deals with the interaction between matter and energy in the
fields of: acoustics, optics, heat, mechanics, electricity, magnetism, radiation, atomic structure, and nuclear
phenomena.
pi (π). The mathematical constant 3.1415927… .
Pi, usually written with the Greek letter π, is an irrational number, a number that cannot be
expressed as an even ratio of any two integers. Pi is the ratio of the length of the circumference of a circle
to the length of its diameter.
piano hinge. A continuous hinge consisting of hinge sections fastened to both the fixed and movable
bodies. A piece of hard spring-steel wire passes through holes in the two sections to act as the hinge pin.
PIBAL (pilot balloon observation). A method of winds-aloft observation by visually tracking a pilot
balloon.
pickling. The treatment of a metal surface with an acid to remove surface corrosion.
pico. The metric prefix meaning one millionth of a millionth (1 x 10-12). A picofarad, for example, is one
millionth of a millionth of a farad. Pico was at one time called micro-micro.
pictorial diagram. A diagram of a wiring or plumbing installation in which sketches of the actual
component, rather than symbols, are used to represent components.
piezoelectric. The characteristic of certain materials that causes them to produce a voltage when they are
bent or twisted or when pressure is applied to them.
piezoelectric crystal. A thin square of quartz or Rochelle salt that has the characteristic of producing a
voltage across its opposite faces when it is physically distorted. The physical resonant frequency of the
crystal makes it useful as a component in an oscillator to maintain a constant electrical frequency. When
the crystal is shocked with a pulse of electrical energy, it vibrates at its resonant frequency. As it vibrates, it
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produces a voltage across its faces that varies at the same frequency as the crystal vibrates.
　
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