This high-voltage traction power line running to Eutingen Railway Substation in Germany radiates electromagnetic waves with very long wavelengths. The lowest commonly encountered radio frequencies are produced by high-voltage AC power transmission lines at frequencies of 50 or 60 Hz. See Figure 2. These extremely long wavelength electromagnetic waves about km! There is an ongoing controversy regarding potential health hazards associated with exposure to these electromagnetic fields E -fields.
Some people suspect that living near such transmission lines may cause a variety of illnesses, including cancer. But demographic data are either inconclusive or simply do not support the hazard theory. Recent reports that have looked at many European and American epidemiological studies have found no increase in risk for cancer due to exposure to E -fields. Extremely low frequency ELF radio waves of about 1 kHz are used to communicate with submerged submarines.
The ability of radio waves to penetrate salt water is related to their wavelength much like ultrasound penetrating tissue —the longer the wavelength, the farther they penetrate. Since salt water is a good conductor, radio waves are strongly absorbed by it, and very long wavelengths are needed to reach a submarine under the surface.
See Figure 3. Figure 3. Very long wavelength radio waves are needed to reach this submarine, requiring extremely low frequency signals ELF. Shorter wavelengths do not penetrate to any significant depth. AM radio waves are used to carry commercial radio signals in the frequency range from to kHz.
The abbreviation AM stands for amplitude modulation , which is the method for placing information on these waves. See Figure 4. A carrier wave having the basic frequency of the radio station, say kHz, is varied or modulated in amplitude by an audio signal. The resulting wave has a constant frequency, but a varying amplitude. A radio receiver tuned to have the same resonant frequency as the carrier wave can pick up the signal, while rejecting the many other frequencies impinging on its antenna.
That audio signal is amplified to drive a speaker or perhaps to be recorded. Figure 4. Amplitude modulation for AM radio. Figure 5. Frequency modulation for FM radio. FM radio waves are also used for commercial radio transmission, but in the frequency range of 88 to MHz. FM stands for frequency modulation , another method of carrying information. See Figure 5. Here a carrier wave having the basic frequency of the radio station, perhaps Since audible frequencies range up to 20 kHz or 0.
Thus the carrier frequencies of two different radio stations cannot be closer than 0. An FM receiver is tuned to resonate at the carrier frequency and has circuitry that responds to variations in frequency, reproducing the audio information.
FM radio is inherently less subject to noise from stray radio sources than AM radio. The reason is that amplitudes of waves add. So an AM receiver would interpret noise added onto the amplitude of its carrier wave as part of the information. An FM receiver can be made to reject amplitudes other than that of the basic carrier wave and only look for variations in frequency.
It is thus easier to reject noise from FM, since noise produces a variation in amplitude. Television is also broadcast on electromagnetic waves. Since the waves must carry a great deal of visual as well as audio information, each channel requires a larger range of frequencies than simple radio transmission.
Other channels called UHF for ultra high frequency utilize an even higher frequency range of to MHz. Note that these frequencies are those of free transmission with the user utilizing an old-fashioned roof antenna. Satellite dishes and cable transmission of TV occurs at significantly higher frequencies and is rapidly evolving with the use of the high-definition or HD format. Calculate the wavelengths of a kHz AM radio signal, a We can rearrange this equation to find the wavelength for all three frequencies.
These wavelengths are consistent with the spectrum in Figure 1. The wavelengths are also related to other properties of these electromagnetic waves, as we shall see.
The wavelengths found in the preceding example are representative of AM, FM, and cell phones, and account for some of the differences in how they are broadcast and how well they travel.
Thus a very large antenna is needed to efficiently broadcast typical AM radio with its carrier wavelengths on the order of hundreds of meters. One benefit to these long AM wavelengths is that they can go over and around rather large obstacles like buildings and hills , just as ocean waves can go around large rocks.
FM and TV are best received when there is a line of sight between the broadcast antenna and receiver, and they are often sent from very tall structures. FM, TV, and mobile phone antennas themselves are much smaller than those used for AM, but they are elevated to achieve an unobstructed line of sight. See Figure 6. Figure 6. The actual antennas are small structures on top of the tower—they are placed at great heights to have a clear line of sight over a large broadcast area.
Astronomers and astrophysicists collect signals from outer space using electromagnetic waves. Even everyday gadgets like our car keys having the facility to lock car doors remotely and being able to turn TVs on and off using remotes involve radio-wave frequencies. In order to prevent interference between all these electromagnetic signals, strict regulations are drawn up for different organizations to utilize different radio frequency bands. One reason why we are sometimes asked to switch off our mobile phones operating in the range of 1.
For example, radio waves used in magnetic resonance imaging MRI have frequencies on the order of MHz, although this varies significantly depending on the strength of the magnetic field used and the nuclear type being scanned. MRI is an important medical imaging and research tool, producing highly detailed two- and three-dimensional images.
Radio waves are broadcast, absorbed, and reemitted in a resonance process that is sensitive to the density of nuclei usually protons or hydrogen nuclei. The wavelength of MHz radio waves is 3 m, yet using the sensitivity of the resonant frequency to the magnetic field strength, details smaller than a millimeter can be imaged.
The intensity of the radio waves used in MRI presents little or no hazard to human health. Microwaves are the highest-frequency electromagnetic waves that can be produced by currents in macroscopic circuits and devices. Microwave frequencies range from about 10 9 Hz to the highest practical LC resonance at nearly 10 12 Hz. Microwaves can also be produced by atoms and molecules.
They are, for example, a component of electromagnetic radiation generated by thermal agitation. The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation.
Figure 7. An image of Sif Mons with lava flows on Venus, based on Magellan synthetic aperture radar data combined with radar altimetry to produce a three-dimensional map of the surface. The Venusian atmosphere is opaque to visible light, but not to the microwaves that were used to create this image. Since it is possible to carry more information per unit time on high frequencies, microwaves are quite suitable for communications.
Most satellite-transmitted information is carried on microwaves, as are land-based long-distance transmissions. A clear line of sight between transmitter and receiver is needed because of the short wavelengths involved. Radar is a common application of microwaves that was first developed in World War II. By detecting and timing microwave echoes, radar systems can determine the distance to objects as diverse as clouds and aircraft. A Doppler shift in the radar echo can be used to determine the speed of a car or the intensity of a rainstorm.
Sophisticated radar systems are used to map the Earth and other planets, with a resolution limited by wavelength. See Figure 7. The shorter the wavelength of any probe, the smaller the detail it is possible to observe. How does the ubiquitous microwave oven produce microwaves electronically, and why does food absorb them preferentially? Microwaves at a frequency of 2. The microwaves are then used to induce an alternating electric field in the oven. Water and some other constituents of food have a slightly negative charge at one end and a slightly positive charge at one end called polar molecules.
The range of microwave frequencies is specially selected so that the polar molecules, in trying to keep orienting themselves with the electric field, absorb these energies and increase their temperatures—called dielectric heating. The energy thereby absorbed results in thermal agitation heating food and not the plate, which does not contain water. Hot spots in the food are related to constructive and destructive interference patterns. Rotating antennas and food turntables help spread out the hot spots.
Another use of microwaves for heating is within the human body. This is used for treating muscular pains, spasms, tendonitis, and rheumatoid arthritis.
Microwaves generated by atoms and molecules far away in time and space can be received and detected by electronic circuits. Deep space acts like a blackbody with a 2.
The microwave and infrared regions of the electromagnetic spectrum overlap see Figure 1. Infrared radiation is generally produced by thermal motion and the vibration and rotation of atoms and molecules.
Electronic transitions in atoms and molecules can also produce infrared radiation. The range of infrared frequencies extends up to the lower limit of visible light, just below red. Night-vision scopes can detect the infrared emitted by various warm objects, including humans, and convert it to visible light. We can examine radiant heat transfer from a house by using a camera capable of detecting infrared radiation. Reconnaissance satellites can detect buildings, vehicles, and even individual humans by their infrared emissions, whose power radiation is proportional to the fourth power of the absolute temperature.
More mundanely, we use infrared lamps, some of which are called quartz heaters, to preferentially warm us because we absorb infrared better than our surroundings. About half of the solar energy arriving at the Earth is in the infrared region, with most of the rest in the visible part of the spectrum, and a relatively small amount in the ultraviolet. On average, 50 percent of the incident solar energy is absorbed by the Earth.
The relatively constant temperature of the Earth is a result of the energy balance between the incoming solar radiation and the energy radiated from the Earth. Most of the infrared radiation emitted from the Earth is absorbed by CO 2 and H 2 O in the atmosphere and then radiated back to Earth or into outer space. Some scientists think that the increased concentration of CO 2 and other greenhouse gases in the atmosphere, resulting from increases in fossil fuel burning, has increased global average temperatures.
Visible light is the narrow segment of the electromagnetic spectrum to which the normal human eye responds. Visible light is produced by vibrations and rotations of atoms and molecules, as well as by electronic transitions within atoms and molecules. The receivers or detectors of light largely utilize electronic transitions.
We say the atoms and molecules are excited when they absorb and relax when they emit through electronic transitions. Figure 8 shows this part of the spectrum, together with the colors associated with particular pure wavelengths. We usually refer to visible light as having wavelengths of between nm and nm. The retina of the eye actually responds to the lowest ultraviolet frequencies, but these do not normally reach the retina because they are absorbed by the cornea and lens of the eye.
Red light has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths. Blackbody radiation from the Sun peaks in the visible part of the spectrum but is more intense in the red than in the violet, making the Sun yellowish in appearance. Figure 8. A small part of the electromagnetic spectrum that includes its visible components.
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Previous Article Does birth order affect personality? Give me additional resources! Show me related lesson plans. As it was explained in the Introductory Article on the Electromagnetic Spectrum , electromagnetic radiation can be described as a stream of photons , each traveling in a wave-like pattern, carrying energy and moving at the speed of light.
In that section, it was pointed out that the only difference between radio waves, visible light and gamma rays is the energy of the photons. Radio waves have photons with the lowest energies. Microwaves have a little more energy than radio waves. Infrared has still more, followed by visible, ultraviolet , X-rays and gamma rays.
The amount of energy a photon has can cause it to behave more like a wave, or more like a particle. This is called the "wave-particle duality" of light. It is important to understand that we are not talking about a difference in what light is, but in how it behaves.
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