Why Does Iron Go Red When Heated?
Iron goes red when heated because its atoms radiate vibratory waves of an electromagnetic nature that are visible as light at a sufficiently high temperature. At 800 degree Celsius the iron is at low-red heat. But as the heat increases the iron will turn bright red, and finally white-hot and molten. Heat is passed through the iron by conduction-the contact of one iron particle with another with no dependence on particle movement.
The heat which is given off as light when iron glows red-hot can be reconverted into heat by the substance on to which it falls. When iron is heated to a temperature below 300 degree Celsius it gives off invisible rays of infra-red radiation which are similar in nature to light. But they do not contain quite enough energy per unit (photon) to stimulate the optic nerve and so be seen by the human eye.
This process is called Incandescence; it is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its temperature. The term derives from the Latin verb incandescere, to glow white. Incandescence is a special case of thermal radiation. Incandescence usually refers specifically to visible light, while thermal radiation refers also to infrared or any other electromagnetic radiation.
In practice, virtually all solid or liquid substances start to glow around 798 K (525 °C) (977 ˚F), with a mildly dull red color, whether or not a chemical reaction takes place that produces light as a result of an exothermic process. This limit is called the Draper point. The incandescence does not vanish below that temperature, but it is too weak in the visible spectrum to be perceivable. At higher temperatures, the substance becomes brighter and its color changes from red towards white and finally blue.
Incandescence is exploited in incandescent light bulbs, in which a filament is heated to a temperature at which a fraction of the radiation falls in the visible spectrum. The majority of the radiation however, is emitted in the infrared part of the spectrum, rendering incandescent lights relatively inefficient as a light source. If the filament could be made hotter, efficiency would increase; however, there are currently no materials able to withstand such temperatures which would be appropriate for use in lamps.
More efficient light sources, such as fluorescent lamps and LEDs, do not function by incandescence. Sunlight is the incandescence of the “white hot” surface of the sun.