Radiation-electromagnetic ultra-violet

Burning stars, such as those originating from Roman candles or shells (Figure 8.5), emit radiation in the ultra-violet, the visible, near-infrared and mid-infrared regions of the electromagnetic spectrum, as displayed in Figure 8.6. 

Electromagnetic radiation of such high energy falls within the Vacuum ultra-violet (VUV), X-ray and y-ray regions of the spectrum. They are called ionizing radiations and their effects on matter are part of the science of radiation chemistry. 

There are numerous properties of materials which can be used as measures of composition, e.g. preferential adsorption of components (as in chromatography), absorption of electromagnetic waves (infra-red, ultra-violet, etc.), refractive index, pH, density, etc. In many cases, however, the property will not give a unique result if there are more than two components, e.g. there may be a number of different compositions of a particular ternary liquid mixture which will have the same refractive index or will exhibit the same infra-red radiation absorption characteristics. Other difficulties can make a particular physical property unsuitable as a measure of composition for a particular system, e.g. the dielectric constant cannot be used if water is present as the dielectric constant of water is very much greater than that of most other liquids. Instruments containing optical systems (e.g. refractometers) and/or electromechanical feedback systems (e.g. some infra-red analysers) can be sensitive to mechanical vibration. In cases where it is not practicable to measure composition directly, then indirect or inferential means of obtaining a measurement which itself is a function of composition may be employed (e.g. the use of boiling temperature in a distillation column as a measure of the liquid composition—see Section 7.3.1). 

Photochromic compounds are those which can be reversibly transformed between two states having different absorption spectra, such change being induced in at least one direetion by the action of electromagnetic radiation (e.g. ultra-violet light). 

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