Passive remote sensing refers to the sensing of electromagnetic waves which did not originate from the satellite or sensor itself. The sensor is just a passive observer. Because microwaves can penetrate haze, light rain and snow, clouds and smoke, these waves are good for viewing the Earth from space. The ERS-1 satellite sends out wavelengths about 5. This image shows sea ice breaking off the shores of Alaska.
Robert Mayanovic, a professor of physics at Missouri State University, said that Doppler radar, which often employs microwaves, is used for air-traffic control and vehicular speed-limit enforcement. When an object is approaching the antenna, the returning microwaves are compressed and thus have a shorter wavelength and higher frequency. Conversely, return waves from objects moving away are elongated and have a longer wavelength and lower frequency.
By measuring this frequency shift, the speed of an object toward or away from the antenna can be determined. Common applications of this principle include simple motion detectors, radar guns for speed-limit enforcement, radar altimeters and weather radar that can track the 3D motion of water droplets in the atmosphere. These applications are called active sensing, because microwaves are transmitted, and the reflected signals are received and analyzed.
In passive sensing, natural sources of microwaves are observed and analyzed. Many of these observations are conducted by satellites looking either back at the Earth or out into space. One of the most common uses of microwaves is to heat food quickly. Microwave ovens are possible because microwaves can be used to transmit thermal energy. The discovery of this phenomenon was purely accidental. In his book, " They All Laughed Spencer, an electronics genius and war hero, was touring one of his laboratories at the Raytheon Company.
Spencer stopped in front of a magnetron , the power tube that drives a radar set. Suddenly he noticed that a candy bar in his pocket had begun to melt.
The first microwave ovens were quite large and expensive, but they have since become so affordable that they are common in homes worldwide. 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 divisions between infrared, visible, and ultraviolet are not perfectly distinct, nor are those between the seven rainbow colors. The figure above shows this part of the spectrum, together with the colors associated with particular pure wavelengths. 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. Colors that can be produced by visible light of a narrow band of wavelengths monochromaticlight are called pure spectral colors.
Quantitatively, the regions of the visible spectrum encompassing each spectral color can be delineated roughly as:. Note that each color can come in many shades, since the spectrum is continuous. The human eye is insensitive to electromagnetic radiation outside this range. By definition any images presented with data recorded from wavelengths other than those in the visible part of the spectrum such as IR images of humans or animals or astronomical X-ray images are necessarily in false color.
An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the midday sky appears blue. The optical window is also called the visible window because it overlaps the human visible response spectrum. This allows visible light to heat the surface. The surface of the planet then emits energy primarily in infrared wavelengths, which has much greater difficulty escaping and thus causing the planet to cool due to the opacity of the atmosphere in the infrared.
Plants, like animals, have evolved to utilize and respond to parts of the electromagnetic spectrum they are embedded in. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product.
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