Physics
Organizer:
- Organized by
-
-
Infrared
-
Infrared
In physics, infrared (IR) or infrared radiation (first discovered in 1800 by astronomer William Herschel) is the electromagnetic radiation with a frequency band of the electromagnetic spectrum lower than that of visible light but greater than that of radio waves, i.e. wavelength between 700 nm and 1 mm (infrared band). The range of Infrared region is 12800 ~ 10 cm-1 and can be divided into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1). This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz and includes most of the thermal radiation emitted by objects near room temperature. The term means “under the red” (from the Latin infra, “under”), because red is the visible color with the lowest frequency.
It is often associated with the concepts of “heat” and “thermal radiation”, since every object with a temperature above absolute zero spontaneously emits radiation in this band (according to Wien’s law increasing the temperature the peak of emission moves more and more towards the visible until the object becomes incandescent).
Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. Infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for the study of these energy states for molecules of the proper symmetry. Slightly more than half of the energy from the Sun arrives on Earth in the form of infrared radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth’s climate. Infrared light is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Incandescent bulbs convert only about 10% of their electrical energy input into visible light energy, while the other 90% is converted to infrared radiation, according to the Environmental Protection Agency.
Infrared rays, invisible to human eye, are emitted by all bodies, at any temperature, with emission that increases with temperature: it is therefore ray energy; they have also the characteristic property to produce considerable heat development when they are absorbed by bodies.
Infrared radiations are diffused by fog, smog, gaseous molecules and are selectively absorbed by many gases in the atmosphere, such as ozone, carbon dioxide and water vapor, to a negligible extent by oxygen and nitrogen. Since infrared radiation is of the same nature as light, the formation of images is altered by the phenomena of diffraction and aberration; in addition, optical systems can be used for them as for visible radiation: lenses, mirrors, prisms, sometimes diffraction gratings.
For the detection of infrared radiation are used different devices and apparatuses: receivers or detectors are transducers that convert the ray energy into electric current or voltage, exploiting the variation of certain physical properties of the detector, or the property of the radiation to impress photographic film. Thermal detectors exploit the variation of physical properties of the detector due to the heating produced by the radiation, such as bolometers, bismuth-silver or copper-constantan thermocouples, thermopiles (succession of several thermocouples with thermoelectric joints in series).
The detectors called photoelectric effect, more sensitive, can be photoelectronic cells, photomultipliers, photovoltaic cells, photoluminescent elements, ie based on a photoluminescent semiconductor crystal, on which the radiation, depending on the material, can have both the effect of stimulating the light emission, resulting in a bright image of the source on a dark background, and to accelerate the decay, resulting in a dark image of the source on a light background. Infrared rays are produced in nature by many sources, including warm-blooded animals, in the technique are obtained by lamps called infrared rays, consisting of an electrical resistance to a temperature of about 600 º C, and equipped with parabolic mirror reflection.
-
1 Reply
-
Technological applications
Industrially infrared rays are used to heat bodies that allow a good penetration of heat inside, for example to dry painted surfaces, or also to heat outdoor environments.
Other important applications include remote temperature measurements in chemical and metallurgical processes, surveys of air pollution or air turbulence, gas analysis, fire alarms, heat map surveys, industrial surveillance with examination of photographic film during manufacture.
In the scientific field, the most interesting applications concern the localization of satellites, devices for the orientation of solar battery instruments, the measurement of temperatures of the Moon, planets or satellites, the measurement of reflected solar radiation, the localization of storm fronts, the visualization of the temperature of land or sea surfaces or ocean currents, the search for oil fields, the measurement of thicknesses of epitaxial films, the determination of the composition of organic substances. The observation of the Universe in the infrared constitutes an entire branch of astronomy, infrared astronomy.
In the military field the most important applications concern the localization of aircraft, ships, submarines, missiles and ground vehicles, surveillance of military targets, night guidance devices.
In the photographic field are sometimes used emulsions sensitive to infrared rays to perform photographs even with haze taking advantage of the property of infrared rays, emitted by the subject to be photographed, to cross almost undisturbed haze and even fog.
The medical applications concern, in the field of diagnosis, the remote measurement of the temperature of the skin and the area below (thermograms), with the possibility of revealing inflammatory, or infectious, or tumor processes; the determination of the content of carbon dioxide in the blood; the measurement of eye size and control of eye movements; the detection of obstacles for the blind. In the therapeutic field is exploited the thermal effect of infrared radiation resulting in vasodilation, acceleration of metabolic exchange and other effects on the peripheral nervous system.