Why is fire colored orange and sometimes blue?

[kirithika]

TL;DR

hi, everyone. this is my first question in pf
yesterday I learned a new term wavelength and I was curious about the color of fire. I was thinking that why is fire colored orange or blue, why couldn't it be in any other color. so can someone kindly explain it

 

[Baluncore]

Welcome to PF.
The colour of the flame is determined by the ionised atoms in the flame. Different elements have different ionisation energy levels, so the photons emitted have different wavelengths and appear to us as different colours.

Throw some common salt, NaCl, into a fire to see the yellow flame of sodium.

There is a list of colours here.
https://en.wikipedia.org/wiki/Flame_test#Common_elements

 

[kirithika]

thanks for the explanation

 

[Physics guy]

Welcome to PF, it is an interesting question. You might have probably seen fire in only blue or orange-yellow color. This is because the most common sources we use are carbon based compounds like kerosene and methane, which give us these colors. In fact the color of the flame depends on which element we are choosing. So when we use compounds of different elements the electrons excite to different energy level and when they return give characteristic colors. For example if you heat Barium compounds it gives Green color corresponding to the wavelength it emits. (Some concepts explained maybe beyond the scope of your grade but it is always good to know some extra facts )

Here is the picture of Barium compound releasing green color flame

 

[Drakkith]

I believe there is also a black-body effect in most common flames as well. The soot and other small particles that aren't completely turned in simple atoms/molecules glow red-orange-yellow because they are hot.

Per wiki's article on candles:

Zone IV - the middle or luminous zone is yellow/ white and is located above the dark zone. It is the brightest zone, but not the hottest. It is an oxygen-depleted zone with insufficient oxygen to burn all of the wax vapor rising from below it resulting in only partial combustion. The zone also contains unburnt carbon particles. Temperature is around 1,200 °C (2,190 °F).

I think the unburnt carbon particles are what glow brightly. 1200 C is hot enough for objects to glow bright yellowish.

 

[Physics guy]

I think the unburnt carbon particles are what glow brightly. 1200 C is hot enough for objects to glow bright yellowish.

Yes true, maybe because our eyes are more sensitive to colours of longer wavelength we see yellow colour to be more luminous

 

[sophiecentaur]

I believe there is also a black-body effect in most common flames as well.

I agree.
Flames are complicated things and their colours have multiple causes, as well as ionic energy levels. If you just look at the effect of a flame on the colour of a heated object ( a clean steel poker, for instance) then the temperature of the object and its colour follow the black body curve. As mentioned above, the yellow colour of a (fuel) gas flame is due to hot Carbon particles. The colour of a clean gas flame, once all the fuel is being burnt, will be 'bluish' but that colour is not part of the black body curve. If it were, the flame would need to be thousands of K. That blue must be caused by energy transitions in the atoms.

But I am not sure what the OP is actually after because of the term "blue", used for "fire" is confusing. I think it has to be to do with Flames in experiments like varying the opening in a Bunsen burner base.

 

[Drakkith]

Yes true, maybe because our eyes are more sensitive to colours of longer wavelength we see yellow colour to be more luminous

Our eyes are most sensitive in the green area of the spectrum. The yellow color comes from the fact that objects at 1200 C give off thermal radiation, with much more light in the red-orange-yellow regions than the green-blue regions. So it looks yellow-orange. Continue to heat an object up to about 2,000 C and enough light in the green-blue region is emitted to mix with the red-orange-yellow light to make the object appear to glow white. This is how an incandescent light bulb works.

 

[kirithika]

thank you all for your replies I finally understood it

 

[Keith_McClary]

understood it

Shin-改