The Journey Of Light From A Flame

Flames emit light through two primary mechanisms. Firstly, small particles of fuel (e.g. wood, coal, or gas) are heated and glow incandescently. Secondly, electrons in excited atoms in the flame change energy stages, emitting light as they stabilise. The colour of a flame depends on several factors, including the type of fuel, the presence of impurities, and the amount of oxygen available during combustion. For example, a candle flame is yellow due to the incandescence of fine soot particles, while a Bunsen burner flame is blue due to the emission of excited molecular radicals.

The role of electrons in light emission

Firstly, let's understand the basic structure of an atom. Electrons orbit the nucleus of an atom in shells or energy levels. These energy levels represent the amount of energy each electron possesses. The lowest energy level is called the ground state, and electrons typically occupy this level. However, various factors can cause electrons to move to higher energy levels, a state known as an excited state.

Now, let's discuss how this relates to light emission in flames. When certain fuels, such as candle wax, are heated, their fuel molecules vaporize and react with oxygen in the air. This exothermic reaction releases heat and light. The high temperature of the flame causes the vaporized fuel molecules to decompose, forming various products, including free radicals.

These free radicals, such as the methylidyne radical (CH) and diatomic carbon (C2), play a crucial role in light emission. The high energy of the flame excites the electrons in these transient reaction intermediates. Exciting electrons means raising them to higher energy levels within the atom. When these excited electrons return to their original energy level, they release the excess energy in the form of light. This process is known as electron de-excitation.

The light emitted during de-excitation has a specific frequency and wavelength, which depend on the energy difference between the initial excited state and the final ground state. This phenomenon is described by the equation E = h*v, where E is the energy of the emitted light, h is Planck's constant, and v is the frequency of the light.

The specific elements and compounds present in the burning material determine the colours observed in a flame. For example, sodium atoms emit an amber yellow colour when heated, while indium produces a blue flame. This colour variation is due to the unique emission spectrum of each element, which arises from the different possible electron transitions within their atoms.

In summary, electrons play a central role in light emission from flames. The excitation and de-excitation of electrons in the transient reaction intermediates formed during combustion result in the emission of light with specific colours and wavelengths. This understanding of the role of electrons in light emission has practical applications in various fields, including chemistry, astronomy, and engineering.

How heat and light are connected

Heat and light are closely related. Light is a form of electromagnetic energy, while heat is a form of kinetic energy contained in the random motion of the particles of a material.

When a flame is hot enough, it emits light. The light we see from a flame is the result of small particles glowing incandescently because they are hot, and from electronic transitions from specific energy levels in excited atoms in the flame. The colour of a flame depends on several factors, including the type of fuel involved in the combustion, the oxygen supply, and the extent of fuel-oxygen pre-mixing.

The light emitted from a flame can be transformed into heat. For example, when sunlight hits a brick wall, the electrons in the atoms in the wall absorb the light, and this absorbed sunlight is turned into thermal energy. The atoms of the brick vibrate sufficiently vigorously that their vibrational energy is roughly equal to the electronic energy (photons) absorbed from the sun. These atoms then make a quantum transition from 'electronically excited' to 'vibrationally excited', meaning that the energy causes the whole atom to move. We feel this motion as heat.

The reverse process is also possible, where materials can absorb electromagnetic radiation, resulting in their particles vibrating more energetically and making the material hotter.

The chemical reaction of fuel and oxygen

Fire is a chemical reaction that occurs between a fuel source and oxygen, which produces light and heat. This process is known as combustion and can be described as the rapid oxidation of a fuel. The fuel must be heated to its ignition temperature for combustion to occur. The reaction will continue as long as there is a sufficient amount of heat, fuel, and oxygen. This is known as the fire triangle.

During combustion, the fuel is heated to such a degree that it releases gases from its surface. These gases are made up of molecules (groups of atoms) that break apart when hot enough, and these fragments then rejoin with oxygen from the air to form new product molecules. The products of combustion are different from the starting materials and are typically water molecules (H2O) and carbon dioxide molecules (CO2). Incomplete combustion can also produce carbon (C) and carbon monoxide (CO). The heat generated by the reaction is what sustains the fire, keeping the remaining fuel at ignition temperature.

The combustion reaction is exothermic, meaning it releases energy in the form of heat and light. The light we see is emitted from flames by two primary mechanisms: small particles glowing incandescently due to high temperatures, and electronic transitions from specific energy levels in excited atoms in the flame, produced as a byproduct of the combustion process. The colour of the flame depends on the substances being burned and any impurities present.

The key ingredient for combustion to occur is the availability of oxygen. The reaction cannot take place in an atmosphere devoid of oxygen. However, oxygen alone is not enough for combustion to occur. The fuel must also be heated to its ignition temperature, and there must be a source of energy to "jump-start" the process, such as a spark or friction.

The colour of a flame

The colder part of a diffusion flame (incomplete combustion) will be red, transitioning to orange, yellow, and white as the temperature increases. The transitions are often apparent in fires, with the colour closest to the fuel being white, then orange above it, and reddish flames the highest of all. A blue-coloured flame only emerges when the amount of soot decreases and the blue emissions from excited molecular radicals become dominant. The blue can often be seen near the base of candles where airborne soot is less concentrated.

The colour blue indicates a temperature hotter than white. Blue flames usually appear at a temperature between 2,600º F and 3,000º F. They have more oxygen and burn hotter because gases burn hotter than organic materials, such as wood. When natural gas is ignited in a stove burner, the gases burn at a very high temperature, yielding mainly blue flames.

The chemical composition of the fuel can also be a factor in the colour of flames. For example, common fossil fuels, such as natural gas and oil, are made up mostly of hydrocarbon compounds, which emit light in the blue spectrum. If other chemical elements are present, they may give off their own unique wavelengths of light when burned. For example, the element lithium will produce a pink flame, while the element tungsten will produce a green flame.

The temperature of a flame

The hottest part of the flame is the base, which usually burns with a different colour to the outer edges or the rest of the flame body. For example, a candle flame burns at around 1400°C at its hottest part, with an average temperature of 1000°C. The colour of a candle flame is typically yellow, which is caused by the incandescence of very fine soot particles produced in the flame. When the air inlet is opened, less soot and carbon monoxide are produced, and the flame becomes blue. The blue colour arises from the emission of excited molecular radicals in the flame, which emit most of their light below 565 nanometers in the blue and green regions of the visible spectrum.

In addition to the type of fuel, other factors that influence the temperature of a flame include atmospheric pressure, oxygen content in the atmosphere, oxidation of the fuel, and distance from the source of the flame.

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