question: How does fire emit light (the sun, a candle, a fire, etc.)?
comment: It's a couple of different things going on, and how much each process contributes to the light produced depends on the type of fire. The first one, which is responsible for most of the light from things like campfires and candles, is called black body radiation. This is the phenomena that all objects convert heat to light. You don't see cold objects light up, because the amount of light and the wavelength (color) of the light depends on the temperature of the object. Colder objects throw off low energy light like radio waves and infra red. Heat them up more and they start glowing red. More and you'll see more yellow, blue, or white light. Fires give off light like this because the gasses and tiny solid particles flying up from the fir are hot enough to glow. The other process is due to the burning process giving the electrons inside of the fuel energy. Electrons in an atom are on a kind of energy ladder, where they can step up and down rungs. The burning process takes an electron and bumps it up the ladder a step (or maybe two or three), then the electron falls back down to where it was before. To do this it has to release all that energy that it just had, which it releases as light. When light is produced in this way, it's only in a very narrow wavelength, which means you see a specific, pure color of light from this process, some of which you're unlikely to see from black body radiation which sort of releases a blend of wavelengths together. This is what you see when you burn copper and the flame is green.
passages: There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight, the radiation emitted by the chromosphere of the Sun at around peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units and roughly 44% of sunlight energy that reaches the ground is visible. Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.
   Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.
   On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible-light energy.
  
  Section::::Regions within the infrared.
   Emitted colors depend on the electronic configuration of the elements involved. Heat energy from the flame excites electrons to a higher quantum level, and the atoms emit characteristic colors (photons with energies corresponding to the visible spectrum) as they return to lower energy levels.
  
  Section::::Campfire colorants.
   In Christianity, from the very first, fire and light are conceived as symbols, if not as visible manifestations, of the divine nature and the divine presence. Christ is the true Light, "and at his transfiguration the fashion Christian of his countenance was altered, and his raiment was white and glistering"; "when the Holy Ghost descended upon the apostles, there appeared unto them cloven tongues of fire, and it sat upon each of them"; "at the conversion of St Paul there shined round him a great light from heaven"; "while the glorified Christ is represented as standing in the midst of seven candlesticks ... his head and hairs white like wool, as white as snow; and his eyes as a flame of fire". Christians are children of Light at perpetual war with the powers of darkness.
   The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a strontium nitrate solution and then inserted into a flame, the strontium atoms emit a red color. Similarly, when copper is inserted into a flame, the flame becomes green. These definite characteristics allow elements to be identified by their atomic emission spectrum. Not all emitted lights are perceptible to the naked eye, as the spectrum also includes ultraviolet rays and infrared lighting.
   Color and temperature of a flame are dependent on the type of fuel involved in the combustion, as, for example, when a lighter is held to a candle. The applied heat causes the fuel molecules in the candle wax to vaporize. In this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. The high temperature of the flame causes the vaporized fuel molecules to decompose, forming various incomplete combustion products and free radicals, and these products then react with each other and with the oxidizer involved in the reaction. Sufficient energy in the flame will excite the electrons in some of the transient reaction intermediates such as the methylidyne radical (CH) and diatomic carbon (C), which results in the emission of visible light as these substances release their excess energy (see spectrum below for an explanation of which specific radical species produce which specific colors). As the combustion temperature of a flame increases (if the flame contains small particles of unburnt carbon or other material), so does the average energy of the electromagnetic radiation given off by the flame (see Black body).
   On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See "Indian atomism".) The basic atoms are those of earth ("prthivi"), water ("pani"), fire ("agni"), and air ("vayu") Light rays are taken to be a stream of high velocity of "tejas" (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the "tejas" atoms.
  
  The "Vishnu Purana" refers to sunlight as "the seven rays of the sun".
   List of light sources
  
  This is a list of sources of light, including both natural and artificial processes that emit light. This article focuses on sources that produce wavelengths from about 390 to 700 nanometers, called visible light.
  
  Section::::Incandescence.
  
  Incandescence is the emission of light from a hot body as a result of its temperature.
  
  BULLET::::- Black-body radiation
  
  BULLET::::- Carbon button lamp (Defunct)
  
  BULLET::::- Earthquake light
  
  BULLET::::- Halogen lamp
  
  BULLET::::- Incandescent light bulb
  
  BULLET::::- Lava
  
  BULLET::::- Nernst lamp (Defunct)
  
  BULLET::::- Volcanic eruption
  
  Section::::Incandescence.:Combustion.
  
  Section::::Incandescence.:Combustion.:Lamps.
  
  BULLET::::- Argand lamp (Defunct)
  
  BULLET::::- Argon flash
  
  BULLET::::- Carbide lamp
  
  BULLET::::- Betty lamp (Defunct)
   Electrical firemaking involves the contact of an electrically heated object to tinder. A current is run through the object until it is red hot, like the burners on an electric stove, and it is brought into contact with the tinder, lighting it. For example, a foil-paper chewing gum wrapper will heat-up and ignite; or a flashlight battery coming into contact with a thin wire mesh (such as steel wool) may produce enough heat to ignite charcloth or other tinder. Larger batteries can generate sparks when its leads touch.
  
  Section::::See also.
  
  BULLET::::- Glossary of firelighting
  
  BULLET::::- Quest for Fire (film)
   The glow of a flame is complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire in Canada is an excellent example of this variation. Near the ground, 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.
   Section::::Creation.
   In contrast, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See "Indian atomism".) The basic atoms are those of earth ("prthivı"), water ("apas"), fire ("tejas"), and air ("vayu"), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms. Light rays are taken to be a stream of high velocity of "tejas" (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the "tejas" atoms. Around the first century BC, the "Vishnu Purana" refers to sunlight as "the seven rays of the sun".
   Section::::Historical theories about light, in chronological order.
  
  Section::::Historical theories about light, in chronological order.:Classical Greece and Hellenism.
   Fire is hot because the conversion of the weak double bond in molecular oxygen, O, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O); the bond energies of the fuel play only a minor role here. At a certain point in the combustion reaction, called the ignition point, flames are produced. The "flame" is the visible portion of the fire. Flames consist primarily of carbon dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.
   Section::::Fuels.:Solid fuels.
  
  The act of combustion consists of three relatively distinct but overlapping phases:
  
  BULLET::::- Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
  
  BULLET::::- Distillation phase or gaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. Flames are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
   If it (the fire) happens by any accident to be put out ... it is not to be lighted again from another fire, but new fire is to be gained by drawing a pure and unpolluted flame from the sunbeams. They kindle it generally with concave vessels of brass, formed by hollowing out an isosceles rectangular triangle, whose lines from the circumference meet in one single point. This being placed against the sun, causes its rays to converge in the centre, which, by reflection, acquiring the force and activity of fire, rarefy the air, and immediately kindle such light and dry matter as they think fit to apply. (tr. Langhorne 1821 1: 195) 
   Lecture 2: Brightness of the Flame - Air necessary for Combustion - Production of Water
  
  Lecture 3: Products: Water from the Combustion - Nature of Water - A Compound - Hydrogen
  
  Lecture 4: Hydrogen in the Candle - Burns into Water - The Other Part of Water - Oxygen
  
  Lecture 5: Oxygen present in the Air - Nature of the Atmosphere - Its Properties - Other Products from the Candle - Carbonic Acid - Its Properties
  
  Lecture 6: Carbon or Charcoal - Coal Gas Respiration and its Analogy to the Burning of a Candle - Conclusion
  
  Section::::External links.
   Section::::Synopsis.:Events of significance.
   Luminous flame
  
  A luminous flame is a burning flame which is brightly visible. Much of its output is in the form of visible light, as well as heat or light in the non-visible wavelengths.
  
  An early study of flame luminosity was conducted by Michael Faraday and became part of his series of Royal Institution Christmas Lectures, "The Chemical History of a Candle".
  
  Section::::Luminosity.
   The sun's rays can be used to produce electrical energy. The direct user of sunlight is the solar cell or photovoltaic cell, which converts sunlight directly into electrical energy without the incorporation of a mechanical device. This technology is simpler than the fossil-fuel-driven systems of producing electrical energy. A solar cell is formed by a light-sensitive p-n junction semiconductor, which when exposed to sunlight is excited to conduction by the photons in light. When light, in the form of photons, hits the cell and strikes an atom, photo-ionisation creates electron-hole pairs. The electrostatic field causes separation of these pairs, establishing an electromotive force in the process. The electric field sends the electron to the p-type material, and the hole to the n-type material. If an external current path is provided, electrical energy will be available to do work. The electron flow provides the current, and the cell's electric field creates the voltage. With both current and voltage the silicon cell has power. The greater the amount of light falling on the cell's surface, the greater is the probability of photons releasing electrons, and hence more electric energy is produced.
   Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion in a fire produces a flame, and the heat produced can make combustion self-sustaining. Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):
   The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.
   Gas lighting
  
  Gas lighting is production of artificial light from combustion of a gaseous fuel, such as hydrogen, methane, carbon monoxide, propane, butane, acetylene, ethylene, or natural gas.
  
  The light is produced either directly by the flame, generally by using special mixes of illuminating gas to increase brightness, or indirectly with other components such as the gas mantle or the limelight, with the gas primarily functioning as a fuel source.
   There are numerous modern applications of fire. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Users of internal combustion vehicles employ fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity.
  