Light Sources and the Particle Nature of Light

Abstract

As the nineteenth century was coming to a close, Newtonian mechanics could explain and predict the motion of objects ranging in size from pebbles to planets while Maxwell’s theory of electromagnetism unified electricity, magnetism, and optical phenomena.

The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote.

—Albert A. Michelson

Glossary of Terms

absolute temperature

Temperature on a scale that begins at absolute zero and measures temperature in kelvins (K). Celsius temperature is converted to absolute temperature by adding 273 K.

blackbody

An ideal absorber and emitter of light.

Bohr model

Model of the atom in which electrons move in stable orbits around the nucleus.

coherent light

Light in which all photons have the same phase and wavelength (such as laser light) .

color temperature

The temperature of the blackbody with the closest spectral energy distribution to a given light source.

efficacy

Measure of the power efficiency (lumens per watt) of a light source.

electron volt

Unit of energy equal to the energy that an electron would acquire traveling from the negative to positive terminal of a 1-volt battery.

emissivity

Ratio of the radiant energy from a nonideal surface to that of a blackbody at the same temperature.

fluorescence

Absorption of light and subsequent reemission at a longer wavelength.

illuminance

Measure of the light intensity striking a surface in lumens/m² (lux) .

irradiance

Measure of the radiation intensity on a surface.

joule

Unit of energy in SI system.

kelvin

One degree on the absolute temperature scale.

laser

Coherent monochromatic light source that utilizes the stimulated emission of radiation.

light-emitting diode (LED)

A very efficient semiconductor light source used especially in outdoor displays, radios, TVs, home appliances, and automobile tail lights.

lumen

Unit for measuring luminous flux or output of light sources.

luminescence

Absorption of energy and emission of light; includes fluorescence and phosphorescence.

luminous flux

Radiant flux scaled according to the sensitivity of the eye at each wavelength.

nanoscale light source

Light-emitting device having dimensions on the order of 10 to 100 nm.

phosphorescence

Absorption of light and delayed emission at longer wavelength.

photometer

Illuminance meter used by photographers, illumination engineers, and others.

photometry

Measurement of energy content in the visible portion of the electromagnetic spectrum.

photon

A particle of light having an energy hc/λ.

population inversion

A condition in which more atoms are in the higher energy state than in the lower one (a nonequilibrium condition).

quantum dot

A nanoscale semiconductor that emits light in wavelengths that are determined by its size.

radiometry

Measurement of radiant energy of electromagnetic radiation.

stimulated emission of radiation

A photon triggers an atom to emit a photon of the same wavelength (as in a laser).

watt

Unit of power in SI system.