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other, since we perceive them differently with our senses (when we can perceive them at all). We see visible light as colour, we feel far-infrared radiation as heat, and so on. But all these energies are sequentially connected to each other as a continuum of waves in the EM spectrum. The nature of the particles depends on how fast they are moving and the qualities that they exhibit. Humans perceive most of the EM frequencies indirectly through their effects, rather than directly perceiving the frequencies themselves. We label and differentiate EM waves from each other according to how they manifest physically. By harnessing the waves with various electrical devices and some passive (non-electrical) materials, we can produce tangible physical phenomena. For instance, we access frequencies on the radio spectrum with an antenna, which transmits and receives radio broadcasts. An X-ray machine utilises certain radiation on the X-ray band, which allows us to see inside the body, and so on. The existence of an EM field includes both electric and magnetic fields. An EM field has certain properties, electrical fields have other properties, and magnetic fields possess yet others. Electrical and magnetic fields can be separated from EM fields as their own distinct energies. They can also exist in EM fields in varying proportions. trough of the wave is the lowest point on bottom. The length of a wave is often measured from peak to peak (see arrow in diagram below). Technically, however, any portion of the wave can be used as a reference point, as long as the measurement addresses one complete cycle (peak to peak, trough to trough, etc.). As the number of waves within a given space—in other words, their frequency—increases in number per second, the size of the waves becomes smaller. And as the number of waves decreases in number per second, the size of the waves becomes larger. Put another way, the higher the frequency or oscillation rate of a wave, the smaller the wavelength. The lower the frequency or oscillation rate of a wave, the larger the wavelength. "A homely comparison to visualize this," Kovacs analogises, "may be a motley army of giants and dwarfs, all under orders to reach the same goal simultaneously; in order to do so the giants step out leisurely, while the dwarfs run and take hundreds of steps for each one of the giants."' In the example below, the frequency of the top wave is higher than the frequency of the bottom wave, because the distance is shorter between the peaks of the waves. The wave forms in this example are simple sine waves. ¢ Frequency, Wavelength and Amplitude All the energies in the EM spectrum have different frequencies. The term frequency pertains to the number of cycles per second (CPS) at which a wave vibrates or moves. (The designation CPS has now been replaced with hertz, or Hz.) Waves also have different sizes or lengths, with various terms such as micron, angstrom, nanometre and metre used to measure the length. (The waves shown in this section are sine waves. Different-shaped waves will be discussed later.) The peak of the wave is the highest point on top. The In order from slower-moving to faster-moving, the frequencies in the EM spectrum include radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays. * Electric Fields and Magnetic Fields So far, | have been discussing electromagnetic radiation from the EM spectrum. Electromagnetic radiation (radiant energy) and electromagnetic fields (non- radiant spaces in which energy exists) operate somewhat differently. Both come from electromagnetic sources. However, energy that radiates exists separately from its source. It travels away from its source, and it continues to exist even if the source is turned off. EM fields are not projected out into space. They no longer exist when the energy source is turned off. Static electricity and magnetism are both static fields that share a complex and intimate relationship with each other. An oscillating electric field generates an oscillating magnetic field, and an oscillating magnetic field generates an oscillating electric field. Each exists at right angles to the other. Most importantly, when movement is introduced to either a static electrical field or a magnetic field, they become electromagnetic fields. This will be important to remember when we later examine a number of different electromedical devices. Key EM Wave Definitions Wave is a movement of energy along a directional axis. Frequency is a rate of oscillation measured by the number of wave cycles per unit time (usually in hertz). Wavelength is the length or distance between two identical points on the wave (which comprises one complete wave cycle). This is described with different terms of measurement, depending on the size of the wave. Amplitude is the point of maximum intensity of the signal (usually regarded as the highest point on the wave). It is comparable to turning up the volume on a radio. Key EM Wave Definitions JUNE - JULY 2010 NEXUS ¢ 27 www.nexusmagazine.com