PhotoGroupy

by Nanette Salvaggio

When the verious forms of radiant energy are placed along a scale of wavelengths, the resulting continuum is called the electromagnetic spectrum. Although each form of radiant energy differs from its neighbors by an extremely small amount, it is useful to divide this spectrum into the generalized categories shown in Figure 1-2. All radiation are believed to be the result of electromagnetic oscillations. In the case of radio waves, the wavelengths are extremely long, being on the order of 1010nm, and are the result of long electrical oscillations. The fact that such energy permeates our environment can easily be demonstrated by turning on a radio or television receiver in any part of the technologically developed world. This form of radiant energy is not believed to have any direct effect upon the human body. Radio waves are customarily characterized by their frequency, expressed in hertz (cycles per second).

The portion of the electromagnetic spectrum that we sense as heat is called the infrared region. The origin of this type of radiant energy, which is shorter in wavelength than radio waves, is believed to be the excitation of electrons by thermal disturbance. When these electrons absorb energy from theri surroundings, they are placed in an elevated state of activity. When they suddenly return to their normal state, electromagnetic radiation is given off. It has been shown that any object at a temperature greater then 273º C will give off this type of radiation. Thus, all the objects we come into contact with give off some infrared energy. In general, the hotter an object, the more total energy it produces and the shorter the peak wavelength. If an object heated to high enough temperature, the wave length of the energy temperature, the wavelength of the energy emitted will become short enough to stimulate the retina of the human eye and cause the sensation of vision. It is this region of the electromagnetic spectrum that is termed light. Notice that it occupies only a narrow section of the spectrum between approximately 380 and 720nm. Because the sensitivity of the human visual system is so low at these limits, 400 and 700nm are generally considered to be more realistic values. Objects with very high temperatures produce ultraviolet energy, which is shorter than 400nm in wavelength. To produce radiant energy shorter in wavelength than about 10nm requires that fast-moving electrons bombard the object. When these rapidly moving electrons strike the object, the sudden stopping produces extremely short wave energy called X-radiation, or more commonly, X-rays. Still-shorter wavelengths can be produced if the electron bombardment intensifies, as occurs in a cyclotron. In addition, when radioactive material decomposes, it emits energy shorter in wavelength than X-rays. In these two cases, the energy is referred to as gamma rays, which are usually 0.000001nm (10 *-6 nm) in wavelength and shorter. These forms of electromagnetic energy are the most energetic, penetrating radiation known. 

Thus it can be seen that the wave theory of radiant energy provides a most useful system for classifying all the known forms of radiation.