Luminescence - How it works



Radiation

Elsewhere in this volume, the term "radiation" has been used to describe the transfer of energy in the form of heat. In fact, radiation can also be described, in a more general sense, as anything that travels in a stream, whether that stream be composed of subatomic particles or electromagnetic waves.

Many people think of radiation purely in terms of the harmful effects produced by radioactive materials—those subject to a form of decay brought about by the emission of high-energy particles or radiation, including alpha particles, beta particles, or gamma rays. These high-energy forms of radiation are called ionizing radiation, because they are capable of literally ripping through some types of atoms, removing electrons and leaving behind a string of ions.

Ionizing radiation can indeed cause a great deal of damage to matter—including the matter in a human body. Even radiation produced by parts of the electromagnetic spectrum possessing far less energy than gamma rays can be detrimental, as will be discussed below. In general, however, there is nothing inherently dangerous about radiation: indeed, without the radiation transmitted to Earth via the Sun's electromagnetic spectrum, life simply could not exist.

The Electromagnetic Spectrum

The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from those with very low frequencies and energy levels, along with correspondingly long wavelengths, to those with very high frequencies and energy levels, with correspondingly short wavelengths.

An electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with mutually perpendicular electrical and magnetic fields that emanate from it. Though their shape is akin to that of waves on the ocean, electromagnetic waves travel much, much faster than any waves that human eyes can see. Their speed of propagation in a vacuum is equal to that of light: 186,000 mi (299,339 km) per second.

PARTS OF THE ELECTROMAGNETIC SPECTRUM.

Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. Though each occupies

MODERN UNDERSTANDING OF LUMINESCENCE OWES MUCH TO MARIE CURIE.
M ODERN UNDERSTANDING OF LUMINESCENCE OWES MUCH TO M ARIE C URIE .
a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply "blurs" into another.

Though the Sun sends its energy to Earth in the form of light and heat from the electromagnetic spectrum, not everything within the spectrum is either "bright." The "bright" area of the spectrum—that is, the band of visible light—is incredibly small, equal to about 3.2 parts in 100 billion. This is like comparing a distance of 16 ft (4.8 m) to the distance between Earth and the Sun: 93 million miles (1.497 · 10 9 km).

When electromagnetic waves of almost any frequency are asborbed in matter, their energy can be converted to heat. Whether or not this happens depends on the absorption mechanism. However, the realm of "heat" as it is most experienced in daily life is much smaller, encompassing infrared, visible, and ultraviolet light. Below this frequency range are various types of radio waves, and above it are ultra high-energy x rays and gamma rays. Some of the heat experienced in a nuclear explosion comes from the absorption of gamma rays emitted in the nuclear reaction.

FREQUENCY AND WAVELENGTH RANGE.

There is nothing arbitrary about the order in which the different types of electromagnetic waves are listed above: this is their order in terms of frequency (measured in Hertz, or Hz) and energy levels, which are directly related. This ordering also represents the reverse order (that is, from longer to shorter) for wavelength, which is inversely related to frequency.

Extremely low-energy, long-wave length radio waves have frequencies of around 10 2 Hz, while the highest-energy, shortest-wavelength gamma rays can have frequencies of up to 10 25 Hz. This means that these gamma rays are oscillating at the rate of 10 trillion trillion times a second!

The wavelengths of very low-energy, low-frequency radio waves can be extremely long: 10 8 centimeters, equal to 1 million meters or about 621 miles. Precisely because these wavelengths are so very long, they are hard to apply for any practical use: ordinary radio waves of the kind used for actual radio broadcasts are closer to 10 5 cm (about 328 ft).

At the opposite end of the spectrum are gamma rays with wavelengths of less than 10 −15 centimeters—in other words, a decimal point followed by 14 zeroes and a 1. There is literally nothing in the observable world that can be compared to this figure, equal to one-trillionth of a centimeter. Even the angstrom—a unit so small it is used to measure the diameter of an atom—is 10 million times as large.

Emission and Absorption

The electromagnetic spectrum is not the only spectrum: physicists, as well as people who are not scientifically trained, often speak of the color spectrum for visible light. The reader is encouraged to study the essays on both subjects to gain a greater understanding of each. In the present context, however, two other types of spectra (the plural of "spectrum") are of interest: emission and absorption spectra.

Emission occurs when internal energy from one system is transformed into energy that is carried away from that system by electromagnetic radiation. An emission spectrum for any given system shows the range of electromagnetic radiation it emits. When an atom has energy transferred to it, either by collisions or as a result of exposure to radiation, it is said to be experiencing excitation, or to be "excited." Excited atoms

COMPARED TO STANDARD INCANDESCENT LIGHT BULBS (SHOWN ON THE LEFT), NEWER FLUORESCENT BULBS (SHOWN ON THE RIGHT) USE FAR LESS ELECTRICITY AND LAST MUCH LONGER. (Photograph by Roger Ressmeyer/Corbis. Reproduced by permission.)
C OMPARED TO STANDARD INCANDESCENT LIGHT BULBS ( SHOWN ON THE LEFT ), NEWER FLUORESCENT BULBS ( SHOWN ON THE RIGHT ) USE FAR LESS ELECTRICITY AND LAST MUCH LONGER . (Photograph by
Roger Ressmeyer/Corbis
. Reproduced by permission.)
will emit light of a given frequency as they relax back to their normal state. Atoms of neon, for instance, can be excited in such a way that they emit light at wavelengths corresponding to the color red, a property that finds application in neon signs.

As its name suggests, absorption has a reciprocal relationship with emission: it is the result of any process where in the energy transmitted to a system via electromagnetic radiation is added to the internal energy of that system. Each material has a unique absorption spectrum, which makes it possible to identify that material using a device called a spectrometer. In the phenomenon of luminescence, certain materials absorb electromagnetic radiation and proceed to emit that radiation in ways that distinguish the materials as either fluorescent or phosphorescent.

Also read article about Luminescence from Wikipedia

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