Thermal radiation electromagnetic spectrum

CONTINUUM THERMAL RADIATION Radiation which involves the transfer of heat by electromagnetic waves, confined to the relatively narrow window of the electromagnetic spectrum (i.e. visible light around 400 nm to infrared light around 1000 nm). The hot particles in and above hrework flames contribute to this type of radiation. 

Thermal radiation has a frequency range principally between 7.5 x 1012s and 1 x 1 O 5 s"1 and, as such, covers most of the visible and infra-red sections of the electromagnetic spectrum (EMS). The relation between thermal radiant energy and temperature is discussed in Volume 1 (Section 9.5.3). 

Analytical applications have been found for all parts of the electromagnetic spectrum ranging from microwaves through visible radiation to gamma (y) rays (Table 1). The emission and absorption of electromagnetic radiation are specific to atomic and molecular processes and provide the basis for sensitive and rapid methods of analysis. There are two general analytical approaches. In one, the sample is the source of the radiation in the other, there is an external source and the absorption or scattering of radiation by the sample is measured. Emission from the sample may be spontaneous, as in radioactive decay, or stimulated by thermal or other means, as in flame photometry and fluorimetry. Both approaches can be used to provide qualitative and quantitative information about the atoms present in, or the molecular structure of, the sample. 

Thermal radiation differs from heat conduction and convective heat transfer in its fundamental laws. Heat transfer by radiation does not require the presence of matter electromagnetic waves also transfer energy in empty space. Temperature gradients or differences are not decisive for the transferred flow of heat, rather the difference in the fourth power of the thermodynamic (absolute) temperatures of the bodies between which heat is to be transferred by radiation is definitive. In addition, the energy radiated by a body is distributed differently over the single regions of the spectrum. This wavelength dependence of the radiation must be taken as much into account as the distribution over the different directions in space. 

FIGURE 7.1 Thermal radiation a region of the electromagnetic-wave spectrum. 

Thermal agitation causes an alkali halide molecule to vibrate about its equilibrium length r0. Since it is then an oscillating dipole, it can interact with electromagnetic radiation. Use (3.1) and the results of Discussion 3.2 to calculate the vibration frequency of the sodium chloride molecule. In what range of the electromagnetic spectrum does this frequency lie ... 

Luminescence, the emission of electromagnetic radiation in excess of that thermally generated, has been observed in several azides. The emission typically occurs in the ultraviolet, visible, or infrared regions of the electromagnetic spectrum. Some external stimulus is required prior to luminescent emission. The stimulus can take the form of photons, energetic particles, electric field, mechanical energy, or energy available from chemical reaction. 

Radiation. Radiant heat transfer occurs in the form of electromagnetic radiation with wavelengths from about 10" to 10" m this is known as thermal radiation. The visible spectrum is centred on wavelengths of 5x 10" m. The total energy flux emitted by a body at an absolute temperature T is given by... 

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