The incandescent metal embers of the spark used to light this Bunsen burner emit light ranging in color from white to orange to red or to blue. This change correlates with their temperature as they cool in the air. The flame itself is not incandescent, as its blue color is due to various other atomic and molecular energy transitions. The blue color comes from the quantized transitions that result from the oxidation of CH radicals.
Incandescence is the emission of light (visible electromagnetic radiation) from a hot body as a result of its temperature. The term derives from the Latin verb incandescere, to glow white.
In practice, virtually all solid or liquid substances start to glow around 798 K (525 °C), with a very dull red color, when no chemical reactions take place that produce 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.
^Dionysius Lardner (1833). Treatise on Heat. Longman, Rees, Orme, Brown, Green & Longman. "The state in which a heated body, naturally incapable of emitting light, becomes luminous, is called a state of incandescence."
^Example 1:'...the stadium positively crackled with the incandescent anger of anguished supporters.' Mark Wilson, 'Rangers 1 Unirea 4', Daily Mail, 21 October 2009 . Example 2: '...there's something very funny about incandescent anger.' Mark Fisher, 'Jerry has a cross to bear', The Scotsman, 5 March 2006 .