Re: Why are lightning bolts and electric sparks blue?

Dear Chris,

Interesting question.  I looked back over previous MadSci responses to 
your question and found one by Prof. Suzanne Willis (1999):

=> http://www
.madsci.org/posts/archives/sep99/936814333.Ph.r.html


Since its been 5 years,  I think its appropriate to expand the answer she 
gave back then.

First thing to consider is the temperature of a lightning bolt.  A number 
of source place this at 28,000-30,000 Kelvin, about 3 times the surface 
temperature of the Sun…

=> http://hypertextb
ook.com/facts/1999/DavidFriedman.shtml

Using this, I have plotted the Planck’s Law for “black body” radiation at 
~29,000K, then applied a Wien’s Law calculation for maximum intensity.

Here are those graphs (Graph Planck and Graph Wien):





The first thing to note is that almost ALL of the light generated in a 
lightning bolt is in the far ultraviolet and “invisible” region.  Very 
little is in the visible region.  Wien’s Law indicates the maximum 
intensity appears at 1028 Angstroms (102.8 nm).  The energy of these 
photons is enough to break apart oxygen and nitrogen molecules.  
Subsequently this light would excite, ionize, and cause the emission of 
their atomic and ionic species.  Both oxygen and nitrogen atoms have part 
of their fluorescence emission spectra in the visible region. All this 
visible emission, taken as a whole, would appear a white or bluish-white 
light to the human eye.

I have read that at a distance, lightning can have different shades of 
color…

=> http://wvlightning.com/faqw10.html

I think this is mainly due to atmospheric effects of light refraction, 
scatter by dust and other atmospheric molecules, etc. Similar to how the 
Sun appears in different shades or hues of color at sunrise (the “green 
flash”) and sunset…

=> http://hyperphysics.phy-
astr.gsu.edu/hbase/atmos/redsun.html


Hope this helps answer your question!

---* Dr. Ken Beck