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light and warmth
Infrared radiation often is confused with warmth but although there is a strong relation they concern two different concepts. Warmth is a physical phenomenon that deals with the temperature of a subject or of a medium that surrounds it, for instance water or air. By convection this warmth can be transferred to the subject, the air or water acting as the transportation medium. In vacuum this
way of transportation is not possible. This is why in space the shadow side of a large and (relative) cold subject remains cold despite the sun is shining unhindered. An object that is heated up for some reason or another, starts emitting infrared radiation and finally, when the temperature reaches values between 500 to 1000 ºC, also light. This infrared radiation behaves like light. It does not require any medium to spread and it propagates rectilinear. An infrared radiator
like the sun is therefore capable of warming a subject in vacuum but only when this subject is exposed to its radiation directly and not when it remains in the shadow. On earth, where there usually is a heat-conducting medium at present, an infrared source will also warm an exposed subject directly. The infrared source, as well as the subject heated by it, will, by convention, transfer a part of their energy to the surrounding medium. This causes a rise of temperature of the medium and that is where the relation between environmental warmth and infrared radiation emerges.
Infrared radiation is not bend by wind or draught and therefore very suitable for warming applications. Infrared radiation in industrial
areas also is used for drying of paint. A drawback here is that the drying time varies, depending on the colour of the paint. White subjects will reflect a great deal of the radiation and they will therefore heat up less easily. Black paint will therefore dry up more quickly than white paint. Other applications are distillation, especially when the fluids to be distilled are very volatile or inflammable, and preparation of food. Infrared radiation will heat op food more
quickly and equable than for instance hot air will do. The same phenomenon is used in therapeutic applications where infrared radiation is used to influence our health in a positive way.
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William Herschel discovered infrared radiation in 1800 by when he tried to measure the amount of energy of different colours of light. In order to do so he refracted light with help of a prism. By holding a glass thermometer in the successive colours he noticed a rise of temperature that increased in the direction of the red part of the refracted light. His attention was attracted by the fact that the increase of the temperature continued beyond the last visible red light and further investigations led to the discovery of infrared radiation.
The infrared part of the electromagnetic spectrum can be divided into three sub-bands, being near-, middle and far infrared, also referred to as IR-A, IR-B and IR-C. The wavelength of near infrared ranges from 700 to 1400 nanometers while far infrared ranges from 15 to 100 micrometers. Far infrared is capable of converting carbohydrates into sugars and is does so in fruits as well as in the human body. In fruit this conversion accompanies the process of maturing. Far infrared radiation only heats the surface of organic materials while the inside of the tissue is mainly heated by conduction. Middle- and, more specific, near infrared penetrates
deeper into the tissue causing a more equable heating. Infrared lamps for (self) medication are aimed on heating the deeper tissues of the skin without reaching the pain barrier, which is about 45 ºC. Far infrared is not capable to perform this task and therapeutic infrared radiators are therefore designed in such a way that the emitted wavelength is somewhere in the near infrared area between 700 and 1400 nanometers (see infrared emission).