The abbreviation AM stands for amplitude modulation—the method for placing information on these waves. A carrier wave having the basic frequency of the radio station for instance, kHz is varied or modulated in amplitude by an audio signal. The resulting wave has a constant frequency, but a varying amplitude. 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. In this case, a carrier wave having the basic frequency of the radio station perhaps Since audible frequencies range up to 20 kHz or 0.
For this reason, 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 because amplitudes of waves add noise. Thus, an AM receiver would interpret noise added onto the amplitude of its carrier wave as part of the information.
An FM receiver can be fashioned to reject amplitudes other than that of the basic carrier wave and only look for variations in frequency. Thus, since noise produces a variation in amplitude, it is easier to reject noise from FM. Electromagnetic waves also broadcast television transmission. However, as 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 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. Microwaves are electromagnetic waves with wavelengths ranging from one meter to one millimeter frequencies between MHz and GHz. Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently with frequencies between MHz 0.
The microwave region of the electromagnetic EM spectrum is generally considered to overlap with the highest frequency shortest wavelength radio waves. As is the case for all EM waves, microwaves travel in a vacuum at the speed of light. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary.
They are used variously between different fields of study see figure. Microwaves overlap with the high frequency portion of the radio section of the EM spectrum. The microwave portion of the radio spectrum can be subdivided into three ranges, listed below from high to low frequencies. Microwaves are the highest-frequency electromagnetic waves that can be produced by currents in macroscopic circuits and devices.
Microwaves can also be produced by atoms and molecules—e. The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation. Since it is possible to carry more information per unit time on high frequencies, microwaves are quite suitable for communications devices.
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. Cosmic Microwave Background : Cosmic background radiation of the Big Bang mapped with increasing resolution.
High-power microwave sources use specialized vacuum tubes to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron used in microwave ovens , klystron, traveling-wave tube TWT , and gyrotron.
Cavity Magnetron : Cutaway view inside a cavity magnetron as used in a microwave oven. Microwaves are used by microwave ovens to heat food.
Microwaves at a frequency of 2. The microwaves then 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 maintain their orientation with the electric field, absorb these energies and increase their temperatures—a process called dielectric heating.
Radar, first developed in World War II, is a common application of microwaves. 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 determine the speed of a car or the intensity of a rainstorm. Sophisticated radar systems can map the Earth and other planets, with a resolution limited by wavelength.
The shorter the wavelength of any probe, the smaller the detail it is possible to observe. A maser is a device similar to a laser, which amplifies light energy by stimulating photons. The maser, rather than amplifying visible light energy, amplifies the lower-frequency, longer-wavelength microwaves and radio frequency emissions. Infrared IR light is EM radiation with wavelengths longer than those of visible light from 0. Distinguish three ranges of the infrared portion of the spectrum, and describe processes of absorption and emission of infrared light by molecules.
Infrared IR light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 0. This range of wavelengths corresponds to a frequency range of approximately GHz to THz, and includes most of the thermal radiation emitted by objects near room temperature.
Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. The infrared part of the electromagnetic spectrum covers the range from roughly GHz 1 mm to THz nm.
It can be divided into three parts: It can be divided into three parts:. Observations of astronomical UV sources must be done from space. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation.
Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum. The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body. As stated above, while infrared radiation is commonly referred to as heat radiation, only objects emitting with a certain range of temperatures and emissivities will produce most of their electromagnetic emission in the infrared part of the spectrum.
However, this is the case for most objects and environments humans encounter in our daily lives. Humans, their surroundings, and the Earth itself emit most of their thermal radiation at wavelengths near 10 microns, the boundary between mid and far infrared according to the delineation above. The range of wavelengths most relevant to thermally emitting objects on earth is often called the thermal infrared. Many astronomical objects emit detectable amounts of IR radiation at non-thermal wavelengths.
Infrared radiation can be used to remotely determine the temperature of objects if the emissivity is known. This is termed thermography, mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs. Applications of IR waves extend to heating, communication, meteorology, spectroscopy, astronomy, biological and medical science, and even the analysis of works of art.
Visible light is the portion of the electromagnetic spectrum that is visible to the human eye, ranging from roughly to nm. Visible light, as called the visible spectrum, is the portion of the electromagnetic spectrum that is visible to can be detected by the human eye. A typical human eye will respond to wavelengths from about to nm 0. In terms of frequency, this corresponds to a band in the vicinity of — THz. A light-adapted eye generally has its maximum sensitivity at around nm THz , in the green region of the optical spectrum.
The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. 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. Visible Spectrum : A small part of the electromagnetic spectrum that includes its visible components. The Innovative Spirit fy As people age into their 60s and beyond, sleep can turn into a nightly disappointment.
What once was peaceful repose becomes fragmented, unsatisfying, or simply evasive. For some, the cause is chronic illness, or the medications they take to treat it. Or, it could be tied to depression and anxiety, the double whammy of aging. Also, some disorders, such as sleep apnea and restless leg syndrome, often worsen in old age. So, with a big chunk of the U. And, a key is finding more efficient and less invasive ways to monitor older adults hoping to stay in their own homes.
Dina Katabi is helping to make that happen. Specifically, it can measure when and for how long a person is spending in different stages of sleep , such as light, deep and REM.
Even with FCC compliance, electronic devices can interfere with one another through radio signals. The FAA Federal Aviation Administration prohibits the use of all electronic devices on airplanes within 10 minutes of takeoff or landing for fear that these devices may interfere with critical navigation and control electronics on the aircraft.
This is a legitimate fear with some aircraft, but not with others, so human factors considerations cause the universal application of the rule. For most electronic devices, FCC compliance is achieved by emitting a minimal amount of radio power. Some devices, however, deliberately emit radio power. These include all devices that use radio waves for communication, including two-way radios of course , radio broadcast, television broadcast, cordless telephones, cellular telephones, microwave links in the telephone network, satellites, garage door openers, and countless other devices.
A major task of the FCC is to ensure that these legitimate emissions of radio signals do not interfere with one another.
To do this, the FCC issues licenses that allow emission of radio signals within a certain frequency range, at a specified power level, within a certain geographic region.
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