ABSTRACT
Radiation heat transfer occurs through the emission of electromagnetic waves or photons from an object. An oscillating electric field generates an oscillating magnetic field. The magnetic field in turn generates an oscillating electric field and so on. These oscillating fields form an electromagnetic wave. Electromagnetic waves are characterized by their frequency, ν, or wavelength, λ, that are related to each other through,
c = λν, (13.1)
where c is the speed of light. Energy of an electromagnetic wave with frequency ν is given by
E hν ν= , (13.2)
where h = 6.626 × 10-34 J.s is the Planck constant. Figure 13.1 shows the electromagnetic wave spectrum. It is seen that the
wavelength of electromagnetic waves can be as low as 10-9 µm and as high as 1010 µm, and their properties and applications are significantly different. Large wavelength radio waves are generated by an electric current that alternates at a radio frequency in an antenna and are used for wireless transmission of information. Microwaves are electromagnetic waves with wavelengths between 0.1 mm and 10 cm and are used in household microwave ovens, short range radio and television broadcasting, cable TV and internet access on coax, radars, wireless protocols such as Bluetooth, and metropolitan area networks such as WiMax. Gamma rays have the shortest wavelength in the electromagnetic spectrum. They are generated by particle interactions in subatomic scales and the energy released through radioactive decays. They have the highest frequency, and therefore the highest energy, among the electromagnetic waves and can penetrate a long distance into materials before being absorbed significantly. X-rays have a longer wavelength and therefore
lower frequency and less energy than gamma rays. They are generated by the collision of accelerating electrons with a metal target. If the electrons have a high enough energy, they can knock out electrons from the inner shells of the metal atoms and create voids. Then, electrons from the higher energy levels will move to these lower energy shells and emit the difference between their original and final energies as X-rays. These waves can penetrate through soft materials such as tissues or blood but are absorbed by harder materials such as bone. This property is used to create X-rays images.
