Why do flames turn blue?
iStockphoto/Thinkstock
High-frequency (bluish) light has high energy quanta and low-frequency
(reddish) light has lower energy quanta. Temperature measures how much
thermal energy is available to go into vibrating particles, etc,
including the particles emitting the light. If the typical thermal
energy of a particle is large compared to a quantum of light of some
color, that color of light is easily emitted. But if the energy quantum
is bigger than the typical thermal energy scale, those quanta hardly
ever come out. So as you heat something up, first the lower energy
(red) quanta show up, then also middle energy (say green), and finally
they’re joined by blue quanta.
This process makes no sense in classical physics, where there's no packet-size for light waves, so it provided the first key to the modern physics of quantum mechanics.
The actual color you see is set by the mixture of different light frequencies. Orange or yellow flames have fairly high wavelengths (low frequency) - most of the light being produced is actually in the infrared range, which we can’t see. Blue-ish flames have much lower wavelengths (high frequency) with a lot of the light off towards the ultraviolet range, which we also can’t see.
When a flame glows white, its temperature is somewhere in between those two. White is what you get when you have all the visible colors mixed together in about the same ratio as sunlight. So a flame with a temperature about the same as the surface of the sun looks white, if there aren’t any chemicals in it which emit any special colors especially easily.
This process makes no sense in classical physics, where there's no packet-size for light waves, so it provided the first key to the modern physics of quantum mechanics.
The actual color you see is set by the mixture of different light frequencies. Orange or yellow flames have fairly high wavelengths (low frequency) - most of the light being produced is actually in the infrared range, which we can’t see. Blue-ish flames have much lower wavelengths (high frequency) with a lot of the light off towards the ultraviolet range, which we also can’t see.
When a flame glows white, its temperature is somewhere in between those two. White is what you get when you have all the visible colors mixed together in about the same ratio as sunlight. So a flame with a temperature about the same as the surface of the sun looks white, if there aren’t any chemicals in it which emit any special colors especially easily.
When you see a flame, you are seeing something that is glowing with a
certain color. Heat naturally causes things to glow. If you heat up a
piece of iron hot enough, it will glow red. Heat it more and it glows
orange or yellow. The temperature controls the color.
In the yellow part of a candle flame, you are seeing tiny particles of soot that are hot enough to glow yellow.
In a blue flame, two things are happening. First, the flame is very hot. Second, it is gas molecules that are glowing rather than pieces of soot. Very hot gas molecules glow blue.
The dominant color in a flame changes with temperature. The photo of the fireplace fire is a good example of this variation. Near the logs, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.
Anytime you see blue in a fire it is hotter than white. The range is between 2,600 and 3,000 degrees Fahrenheit and its the most oxygen-rich type of flame. A bunsen burner is a good example:
Bunsen burner flames:
1) air hole closed
2) air hole slightly open
3) air hole half open
4) air hole almost fully open (this is the roaring blue flame).
Bunsen
burners use a mixture of gases. Gas burns hotter than organic materials
such as wood and straw. Natural gas stove flames are blue. Propane
flames are blue with yellow tips. The hottest fires are from
oxyacetylene torches (about 3000 degrees Centigrade) that combine oxygen
and gas to create pinpoint blue flames.
Color
also tells us about the temperature of a candle flame. The inner core
of the candle flame is light blue, with a temperature of around 1800 K
(1500 °C). That is the hottest part of the flame. The color inside the
flame becomes yellow, orange, and finally red. The further you get from
the center of the flame, the lower the temperature will be. The
brightest red portion is around 1070 K (800 °C).
The round blue flame is a photo of a candle burning experiment in the International Space Station. Candle flames on earth have several different temperatures within the flame due to the variations caused by convection flows. In the zero gravity of the space station the flame burns rounder, slower, hotter and more blue.
Our traditional associations for color and temperature tell us that red is hot and blue is cold. How hard is it to think of blue as a hot color?
Traditional
color theory says that warm colors advance and cool colors recede. In
my experience, this is only true when the colors are the same
saturation. If you have a pure red and a pure blue – the red advances.
If you have a brick red and a bright turquoise blue – the blue
advances. Higher saturation trumps warmth every time.
In the yellow part of a candle flame, you are seeing tiny particles of soot that are hot enough to glow yellow.
In a blue flame, two things are happening. First, the flame is very hot. Second, it is gas molecules that are glowing rather than pieces of soot. Very hot gas molecules glow blue.
If you look into a wood fire, then up in
the night sky, you might see the same colors in the flames as you see in
the stars. But is there a correlation between these fire colors and the
colors of the stars?
The colors of stars indicate their
temperatures. Blue-white Vega is hotter than red Aldebaran. Star colors
stem from “black-body radiation”, the same sort of radiation you see in
metal heated to red, orange, or white heat. The orange glow seen between
logs in the heart of a fire is also black-body radiation
But the orange seen in the actual tongues
of flame is not. Instead, the colors of flames in a wood fire are due to
different substances in the flames. The bright orange of most wood
flames is due to the presence of sodium, which, when heated, emits light
strongly in the orange. The blue in wood flames comes from carbon and
hydrogen, which emit in the blue and violet. Copper compounds make green
or blue, lithium makes red.
The dominant color in a flame changes with temperature. The photo of the fireplace fire is a good example of this variation. Near the logs, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.
The temperature range from Red to White:
- Red
- Just visible: 525 °C (980 °F)
- Dull: 700 °C (1,300 °F)
- Cherry, dull: 800 °C (1,500 °F)
- Cherry, full: 900 °C (1,700 °F)
- Cherry, clear: 1,000 °C (1,800 °F)
- Orange
- Deep: 1,100 °C (2,000 °F)
- Clear: 1,200 °C (2,200 °F)
- White
- Whitish: 1,300 °C (2,400 °F)
- Bright: 1,400 °C (2,600 °F)
- Dazzling: 1,500 °C (2,700 °F)
Anytime you see blue in a fire it is hotter than white. The range is between 2,600 and 3,000 degrees Fahrenheit and its the most oxygen-rich type of flame. A bunsen burner is a good example:
Bunsen burner flames:
1) air hole closed
2) air hole slightly open
3) air hole half open
4) air hole almost fully open (this is the roaring blue flame).
Bunsen
burners use a mixture of gases. Gas burns hotter than organic materials
such as wood and straw. Natural gas stove flames are blue. Propane
flames are blue with yellow tips. The hottest fires are from
oxyacetylene torches (about 3000 degrees Centigrade) that combine oxygen
and gas to create pinpoint blue flames.
Color
also tells us about the temperature of a candle flame. The inner core
of the candle flame is light blue, with a temperature of around 1800 K
(1500 °C). That is the hottest part of the flame. The color inside the
flame becomes yellow, orange, and finally red. The further you get from
the center of the flame, the lower the temperature will be. The
brightest red portion is around 1070 K (800 °C).The round blue flame is a photo of a candle burning experiment in the International Space Station. Candle flames on earth have several different temperatures within the flame due to the variations caused by convection flows. In the zero gravity of the space station the flame burns rounder, slower, hotter and more blue.
Our traditional associations for color and temperature tell us that red is hot and blue is cold. How hard is it to think of blue as a hot color?
Traditional
color theory says that warm colors advance and cool colors recede. In
my experience, this is only true when the colors are the same
saturation. If you have a pure red and a pure blue – the red advances.
If you have a brick red and a bright turquoise blue – the blue
advances. Higher saturation trumps warmth every time.