Luminous and Non-Luminous Flames
Date: September 2006
Why does a luminous flame appear yellow and a
nonluminous flame appear blue?
The yellow color in a luminous flame is basically the black-body
emission from hot particles of soot in the flame. they are hot, and
they glow like the filament of a light bulb.
In a blue flame, there are no particles of soot to give that
incandescent radiation. Instead, the main color you see is blue
emission from the high-energy C2 molecule.
The frequency or wavelength of light released by atoms or molecules
depend on the energy being released by those substances. The Planck
Equation describes exactly that, that there is a specific energy to
a specific wavelength or frequency.
The difference between a yellow and blue flame depend on the amount
of oxygen that is mixed in with the fuel. More oxygen means that the
combustion reaction is more efficient and more energy is released.
Remember that the more blue the visible light appears, the more
energy that light will have.
Thus, it should make sense now that when more oxygen is fed into the
system, the combustion is more efficient, more energy is released,
and the light will be on the more energetic end of the spectrum, blue.
Greg (Roberto Gregorius)
Click here to return to the Chemistry Archives
Update: June 2012