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A brilliant green coloring of the upper edge of the sun as it appears at sunrise or disappears at sunset when there is a clear, distinct horizon. It is due to refraction by the atmosphere, which disperses the first (or last) spot of light into a spectrum and causes the colors to appear (or disappear) in the order of refrangibility. The green is bent more than red or yellow and hence is visible sooner at sunrise and later at sunset.

Related Terms

POLARIZING PYROMETER

A type of pyrometer, such as the Wanner optical pyrometer, in which monochromatic light from the source under investigation and light from a lamp with filament maintained at a constant but unknown temperature are both polarized and their intensities compared.

PLANISHING

Smoothing the surface of a metal by a rapid series of overlapping, light hammerlike blows or by rolling in a planishing mill.

PLAIN CONCRETE

Concrete without reinforcement but often with light steel to reduce shrinkage and temperature cracking.

PHOTONICS

The electronic technology involved with the practical generation, manipulation, analysis, transmission, and reception of electromagnetic energy in the visible, infrared, and ultraviolet portions of the light spectrum. It contributes to many fields, including astronomy, biomedicine, data communications and storage, fiber optics, imaging, optical computing, optoelectronics, sensing, and telecommunications. Also known as optoelectronics.

PHOTONEPHELOMETER

A nephelometer that uses a photocell or phototube to measure the amount of light transmitted by a suspension of particles.

PHOTOVOLTAIC EFFECT

The production of a voltage in a nonhomogeneous semiconductor, such as silicon, or at a junction between two types of material, by the absorption of light or other electromagnetic radiation.

PHOTOTUBE CURRENT METER

A device for measuring the speed of water currents in which a perforated disk, which rotates with the current by means of a propeller, is placed in the path of a beam of light that is then reflected from a mirror onto a phototube.

PHOTOELECTRIC LIQUID-LEVEL INDICATOR

A level indicator in which rising liquid interrupts the light beam of a photoelectric control system; used in a tank or process vessel.

PHOTOFLASH BOMB

A missile dropped from aircraft; it contains a photoflash mixture and a means for ignition at a distance above the ground, to produce a brilliant light of short photoelectric effect See photoelectricity.

PHOTOELECTRIC TURBIDIMETER

Device for measurement of solution turbidity by use of photocells to detect the loss of intensity of light beamed through the solution.

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