Light: The Physics of Art and Visual Perception: Flame The Physics of Art and Visual Perception

InDepth Material: Flames

Fire flames produce light from a mixture of atomic and molecular de-excitation, and the heating of soot particles. The soot particles are so small that they have quantized vibration states, so they do not radiate as efficiently as "blackbodies". Here are some WWW sites:
Candle flame parts and description


The spectrum of the yellow-white portion of a candle flame.

The yellow-white part is glowing soot. At this resolution, it has a pretty continuous (all visible wavelengths) spectrum, although it is certainly fainter in the blue. The camera CCD only responds to visible light, so the picture does not show the infrared wavelengths. This spectrum is not really a "blackbody" spectrum, although it is nearly so. The soot particles are too small to radiate efficiently in the far infrared and not all visible wavelength frequencies are available for particle vibration modes.
Candle flame colors


The spectrum of a propane flame on a stove.

Notice two blue wavelength images, one green wavelength image, and a faint red wavelength image. Here we are looking at single wavelength radiations from various electron transitions in the excited molecules formed in the flame.
Different colors from different chemicals

Experiments:
1. Try looking at a candle flame in a dark room with the grating in the LabKit. Put the grating over one eye and close the other. The spectrum should appear on both sides away grom the flame. Look from far enough away so the flame spectrum colors are distinct.
2. Put various solid chemicals (salt, sugar, alum, etc) on a fine-mesh screen in the flame from a gas stove to see the colors.

Sample Questions:

What is responsible for the blue portion of a candle flame?

What is responsible for the yellow portion of a candle flame?

Why do different chemicals make different colors when burned?