What Is Radiation?
The word radiation usually refers to electromagnetic radiation. Electromagnetic radiation is energy that spreads through space or through various types of matter in the form of waves. Radiation is emitted by many different sources that are man-made (e.g. antennas/base stations) or natural (e.g., the sun). Electromagnetic radiation is a combination of electric and magnetic fields – electromagnetic fields – created by charges and electric currents. Electromagnetic radiation is described by physical attributes that include frequency, wave length and indicating the intensity of radiation. Matter absorbs electromagnetic radiation in the form of tiny particles of energy called photons. In this manner, in our visual system, cells in the retina of the eye absorb photons of visible light , and translate them into images. The magnitude of absorption of electromagnetic radiation by matter depends both on the attributes of the electromagnetic radiation and on those of the absorbing matter.
Basic principles/concepts in radiation
Electromagnetic radiation is characterized by the following basic physical attributes:
Frequency – the number of waves per unit of time, usually one second. The unit of measurement is the Hertz (Hz) – I Hz= 1/sec. Frequency, depending on the context, is expressed in multiples of the number of waves per second; e.g. Kilohertz (103 Hz, kHz, 1000 Hz), Megahertz (106 Hz, MHz, 1 million Hz), and Gigahertz (109 Hz, GHz, 1 billion Hz).
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1012Hz=Terahertz (THz) |
1Hz=100 Hertz (Hz) |
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1015Hz=Petahertz (PHz) |
103Hz=Kilohertz (kHz) |
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1018Hz=Exahertz (EHz) |
106Hz= Megahertz (MHz) |
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109Hz=Gigahertz (GHz) |
Wave length – expresses the distance between two successive wave peaks. The units of measure are: Kilometer (Km = 1000 m), meter (m), centimeter (cm = 10-2 m), millimeter (mm = 10-3 m), micron (µm = 10-6 m), nanometer (nm = 10-9 m), and Ångstrom (Å = 10-10 m).
Wave amplitude – indicates the strength of the electric and magnetic fields of radiation. The electric field is measured in units of volts/meter (V/m), and the magnetic field in units of amperes/meter (A/m) or in milligauss units (mG), or Tesla (T).
Radiation intensity– denotes the amount of radiation (energy) emitted from a radiating source per unit of time. Radiation intensity is expressed in Watts (W), milliwatts (mW = 10-3 W), microwatts (µW = 10-6 W) and nanowatts (nW= 10-9 W).
Electromagnetic field
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The electromagnetic field describes the basic attributes characterizing electromagnetic radiation. Electric fields are formed around bodies carrying an electric charge. The greater the distance from the charge, the weaker the field. Friction between two bodies, for instance, creates electric charges on these bodies, generating electric fields around them. These electric fields attract dust, floating particles, etc. The electric field is measured in units of volts per meter (V/m).
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Pattern of lines of the electric field
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Pattern of lines of a magnetic field |
Magnetic fields are created around an electric current (such as that flowing through high-voltage power lines). The stronger the current, the stronger the magnetic field created around it. The power of the field decreases as a square of the distance from the source of the electric current. Magnetic fields are also generated around magnetic bodies. For instance, if iron filings are scattered near a magnet, they will align themselves according to the field lines of the magnet. The magnetic field is measured in units of Amperes per meter (A/m) or milligauss units (mG) or Tesla (T). |
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Scattered iron filings near magnets illustrate the magnetic field lines
Sources of electromagnetic fields
Electromagnetic fields arise from different sources, some natural and some man-made.
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Natural sources of electromagnetic fields Electromagnetic fields are created in nature, for instance, when electric charges accumulated in clouds break apart in electric bursts in the form of lightning. An example of a natural magnetic field is the earth's magnetic field (also termed magnetosphere). This field is created by the spinning of the (liquid) metal core of the earth about its own axis. The earth's magnetic field is static and weak, and may be detected using a compass. Its strength is 0.25-0.65 Gauss, depending on the geographic location on earth. |
The earth's magnetic field |
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Artificial sources of magnetic fields Electric and magnetic fields may be created artificially. For example, an electric battery carries a fixed electric field between its two poles (terminals). When a closed circuit is formed between the two terminals an electric current flows between them initiating the formation of a magnetic field around the conductor that connects the two poles. |
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Waves of electromagnetic fieldsElectromagnetic fields may also spread through space in waves having physical attributes such as frequency, wave length and wave amplitude. In a vacuum the electromagnetic waves travel at a rate of about 300.000 km/sec (the speed of light). For example, television and radio broadcasting stations and cellular network base stations use a range of frequencies named radio waves (or radiofrequency). Visual or audio signals are converted into signals of electromagnetic waves that are broadcast through antennas, spread out in space, then received by receiver antennas, and converted again into visual or audio signals by the television, radio, mobile phone, etc. Light reaching us from the sun is composed of electromagnetic field waves. However, light waves have far higher frequencies than radio waves (see the electromagnetic spectrum). References
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 Electromagnetic wave |
24.9.2015
