|MadSci Network: Physics|
Daryl, Light and radiant infrared heat are both forms of electromagnetic radiation. Radiant heat is often referred to as "infrared" radiation, because it is invisible and "below" the red that we see. Other forms of electromagnetic radiation include x-rays, ultraviolet rays, radio and television signals. Each form of radiation can be characterized by its wavelength, which is the distance between the peaks of the electromagnetic wave. (This is similar to measuring the distance between the peaks of an ocean wave, except that you canít touch or see the electromagnetic wave, itself.) Almost all things give off electromagnetic radiation. The radiation given off by an object is related to its temperature. The higher the temperature, the shorter the wavelength. X-rays have the shortest wavelength, followed by ultraviolet, then visible light, then infrared, and finally radio waves. The collection of all wavelengths is known as the "spectrum". There is a nice web encyclopedia entry that describes infrared radiation at: this site Incandescent lamps (which you would know as standard light bulbs) work by heating up a material, called a filament, to a very high temperature. The high temperature is needed in order for it to emit radiation in the visible light part of the spectrum (remember that we said the wavelength is shorter for higher temparture). These bulbs are "optimized" so that they give off as much energy as possible in the visible part of the spectrum. However, in addition to light, standard light bulbs also give off significant energy in the infrared part of the spectrum, which you would know if you have ever burned yourself on a light bulb. This is because objects give off energy in a whole range of wavelengths, not just a small portion. In a "heat lamp", or infrared lamp, the filament is optimized to give off most of its energy in the infrared portion of the spectrum, rather than the visible light portion. Optimization is usually done through selection of the filament material. So an infrared lamp might use quartz, for example, instead of tungsten that is used in light bulbs. An infrared lamp will sometimes have a slight orange or red glow to it, but not be very bright. That is why you might think of a heat lamp as a "red" light bulb. (These are different from darkroom "red" lights used for photography.) If you have ever looked at an electric stove element that is on high, you will sometimes see that when it gets very hot, it starts to glow orange. This is because at the higher temperature, it starts to give off visible light. Again, the stove is optimized for heat, not light, so the glow is very dull. Another web site that you might look into contains notes from a college course in physics. Donít be worried, though, it is quite easy to understand. Here is the link: http://debye.colorado.edu/phys1230/ Good luck with your investigation! [note added by MadSci Admin: Some, especially older or less expensive "heat" lamps are actually just regular spot (or flood) lamps that have a red filter on them that blocks a lot of the visible light and allows the longer wavelength infrared energy through.]
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