THERMAL RADIATION

1) The electromagnetic spectrum includes cosmic waves, gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, thermal radiation, microwaves, radio waves and electrical power wave

a) Gamma rays are produced by nuclear reactions,

b) X-rays by the bombardment of metals with high-energy electrons,

c) Microwaves by special types of electron tubes such as klystrons and magnetrons

d) Radio waves by the excitation of some crystals or by the flow of alternating current through electric conductors.

2) The type of electromagnetic radiation that is pertinent to heat transfer is the thermal radiation emitted as a result of energy transitions of molecules, atoms, and electrons of a substance. Thermal radiation is continuously emitted by all matter whose temperature is above absolute zero.

3) Thermal radiation is also defined as the portion of the electromagnetic spectrum that extends from about 0.1 μm to 100 μm hence thermal radiation includes the entire visible and infrared (IR) radiation as well as a portion of the ultraviolet (UV) radiation.

a) Light is simply the visible portion of the electromagnetic spectrum that lies between 0.40 μm and 0.76 μm. Bodies start emitting noticeable visible radiation at temperatures above 800 K. The tungsten filament of a light bulb must be heated to temperatures above 2000 K before it can emit any significant amount of radiation in the visible range

b) The radiation emitted by bodies at room temperature falls into the infrared region of the spectrum, which extends from 0.76 μm to 100 μm

c)  The ultraviolet radiation includes the low-wavelength end of the thermal radiation spectrum and lies between the wavelengths 0.01 μm and 0.40 μm

4) The electromagnetic radiation emitted by the sun is known as solar radiation, and nearly all of it falls into the wavelength band 0.3 μm – 3 μm. Almost half of solar radiation is light about 12 percent of solar radiation is in the ultraviolet range and remaining being infrared.

5) Radiation is constantly emitted, as well as being absorbed or transmitted throughout the entire volume of matter. That is, radiation is a volumetric phenomenon. However for opaque (nontransparent) solids such as metals, wood, and rocks, radiation is considered to be a surface phenomenon, since the radiation emitted by the interior regions can never reach the surface, and the radiation incident on such bodies is usually absorbed within a few microns from the surface.