What is the color of a flame
Why does a candle flame glow bluish below and reddish above?
Zones of a candle flame
If you take a closer look at a candle flame, you will notice that different zones can be made out in it. While the actual flame shines in shades of yellow and orange, there are also blue shining regions at the edges, near the wick. In contrast, there is a dark area directly above the wick. In addition to the colors, the temperatures inside the flame also vary: Above the wick it is just 600 degrees Celsius, while in the outer areas of the yellow flame it is up to 1400 degrees Celsius.
The fuel for the candle is wax, for example paraffin. If you light the candle, the wax around the wick melts, rises in the wick due to capillary action and evaporates due to the heat of the flame. First of all, the wax vapor is not yet sufficiently heated and collects in a dark, cooler zone around the wick. As the temperature increases towards the flame, the long hydrocarbon chains in the paraffin (CnH2n + 2) and sometimes combine to form new molecules. Finally, these compounds of carbon and hydrogen are drawn towards the flame and mix with oxygen on the surface.
In this so-called reaction zone, the various molecules react both with one another and with the oxygen, releasing heat. Ultimately, it is true that primarily water vapor and carbon dioxide are released into the environment, but the complex reactions produce a large number of intermediate products. Including molecular carbon (C.2) and CH molecules, which, through so-called chemiluminescence, provide the bluish light near the wick of the candle: They are already excited from a chemical reaction, i.e. one of their electrons occupies a higher energy level than in the ground state. A little later these molecules give off their excess energy in the form of light.
While atoms only emit very specific wavelengths, these are distributed over a larger range of wavelengths in the case of molecules. The strongest emission of the CH molecule is at a wavelength of 432 nanometers, which corresponds to a blue color. The C2-Molecules deliver several blue and green tones at 436, 475 and 520 nanometers, the so-called swan bands. Taken together, the lower area of the reaction zone shines in a blue light. In fact, this zone extends over the entire edge of the flame - as long as the candle does not soot - but apart from the dark region near the wick, the pale blue light is outshone by the significantly brighter yellow emission.
Radiation spectra for different temperatures
This intense radiation is due to soot particles: carbon-rich molecules form in the reaction zone, which eventually grow into larger graphite particles with up to several million carbon atoms. Due to the prevailing temperatures of around 1200 degrees Celsius, the soot particles begin to glow and give off light - just like the filament in an incandescent lamp. In contrast to atoms or molecules, the solid particles not only show individual colors, but a continuous spectrum.
The emission spectrum of the soot particles corresponds approximately to that of a black body: the intensity of the light at different wavelengths is determined solely by the temperature of the flame. At 1200 degrees Celsius the maximum intensity is 1970 nanometers and thus in the infrared range, but red to yellow wavelengths dominate in the visible spectral range - the typical colors of a candle flame.
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