Ultraviolet Radiation

a form of radiation that occurs beyond the violet end of the visible light spectrum of ELECTROMAGNETIC WAVES. Ultraviolet rays have a FREQUENCY ranging from 10¹⁵Hz to 10¹⁸Hz, with a wavelength from 10^-7m to 10^-10m. They are part of natural sunlight and are also emitted by white-hot objects (as opposed to red-hot objects, which emit INFRARED RADIATION). As well as affecting photographic film and causing certain minerals to fluoresce, ultraviolet radiation will rapidly destroy bacteria. Although ultraviolet rays in sunlight will convert steroids in human skin to vitamin D (essential for healthy bone growth), too much can cause irreversible damage to the skin and eyes and damage the structure of the DNA in cells. Fortunately, a great deal of the ultraviolet radiation from the sun is prevented from reaching the earth as the OZONE LAYER in the upper atmosphere acts as a UV filter.

Taken from Dictionary of Science

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The discovery of UV radiation was intimately associated with the observation that silver salts darken when exposed to sunlight. In 1801 the German physicist Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum were especially effective at darkening silver chloride-soaked paper. He called them "de-oxidizing rays" to emphasize their chemical reactivity and to distinguish them from "heat rays" at the other end of the visible spectrum. The simpler term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century. The terms chemical and heat rays were eventually dropped in favor of ultraviolet and infrared radiation, respectively.

The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is strongly absorbed by air, was made in 1893 by the German physicist Victor Schumann.

Taken from Wikipedia

Radioactivity

the giving out of particles (known as α or β particles and γ rays) by unstable substances that are disintegrating.

Taken from Dictionary of Science

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Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a stochastic process on the atomic level, in that it is impossible to predict when a given atom will decay, but given a large number of similar atoms the decay rate, on average, is predictable.

The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 10⁹ decays per second) or TBq (terabecquerel, 1 x 10¹² decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 10¹⁰ Bq. The use of Ci is presently discouraged by the SI.

Taken from Wikipedia

Background

'interference' which affects the reading of a signal that is being measured. The interference signals come from sources such as natural RADIOACTIVITY and COSMIC RAYS and must be allowed for when measurements are being taken.

Taken from Dictionary of Science

Aberration

is what happens in physics when a lens (or curved mirror) does not produce a true image. It is caused by the light rays from any point on the object being viewed forming an image (focusing) at slightly different positions. In astronomy, aberration refers to the apparent change in position of a star. This is due to the fixed speed of light travelling from the star and the movement of the person looking at the star, because the Earth is moving in its orbit.

Taken from Dictionary of Science