Avaliable with permission from the University of Wisconsin Press, and author Bassam Shakashiri.

The brightest part of a candle flame appears yellow. Spectroscopic examination of the flame helps to determine what species are responsible for this emission.

Three options—a diffraction grating (Procedure A), a compact disc (Procedure B), and a video system with either of these diffraction techniques (Procedure C)—are presented for observing the visible emission spectrum of a candle flame. The spectrum appears to be continuous. A white porcelain dish held in the luminous part of the flame collects a deposit of black soot.

MATERIALS FOR PROCEDURE A

• handheld, single-axis transmission diffraction gratings, one for each audience member (See Procedure A for description.)
• candle with exposed flame, e.g., a taper
• candle stand
• matches
• white porcelain dish at least 75 mm in diameter
  

MATERIALS FOR PROCEDURE B

• compact discs, one for each audience member
• candle with exposed flame, e.g., a taper
• candle stand
• matches
• white porcelain dish at least 75 mm in diameter
  

MATERIALS FOR PROCEDURE C

• video projection system*
• handheld, single-axis transmission diffraction grating mounted as a slide, or a compact disc
• candle with exposed flame, e.g., a taper
• candle stand
• matches
• white porcelain dish at least 75 mm in diameter
  * Various types of suitable video projection systems are described on page xxxiii. Some video projection systemsare not capable of displaying a continuous spectrum but instead show a spectrum of several distinct colors. Test the system before using it to be sure that a continuous spectrum is displayed. 88 12.1 The Emission Spectrum from a Candle Flame
  

PROCEDURE A

Preparation

Each member of the audience should have a single-axis transmission diffraction grating. This grating material is available as a thin, flexible plastic sheet and needs to be mounted in a rigid holder for convenient use. Suitable gratings are available already mounted as 2-in × 2-in slides. Alternatively, bulk grating material can be used to prepare suitable gratings. A simple way to do this is to use a hole punch to make a round hole in an index card and tape a 1-cm × 2-cm piece of grating material over the hole (see Figure 1). (The audience members may also be instructed in how to make these themselves as part of the presentation.)
Figure 1. Piece of grating material taped over hole punched into an index card.

Presentation

This demonstration needs to be done in a room that can be darkened as completely as possible, including closing blinds, shutting doors, and so on.
Distribute the handheld transmission diffraction gratings to the audience.
Place a candle where it is easily visible to all members of the audience. Light the candle. Instruct the audience members to look through the diffraction gratings directly at the candle flame. The full spectrum of colors from the flame will be seen to either side of the flame. If the spectrum appears above and/or below the flame image, rotate the grating by 90 degrees. Darken the room. The spectrum appears continuous, showing all colors from red, through orange, yellow, green, and blue, to violet.
With the lights on, display the white porcelain dish to the audience, showing that the outer surface is clean. Hold the bottom of the dish in the white region near the top of the flame for a few seconds. Show the audience the black spot of soot that has deposited on the bottom of the dish.
Extinguish the candle.

PROCEDURE B

Preparation and Presentation

This demonstration needs to be done in a room that can be darkened as completely as possible, including closing blinds, shutting doors, and so on.

Distribute compact discs to the audience.

Instruct the audience members to hold the compact disc in a position so that they see the reflection of the candle flame on the shiny side of the disc. Darken the room. When the white reflection of the flame is visible, the spectrum of the flame can be viewed to either side of the white flame. They may need to tip the disc a bit to see the spectrum. The spectrum appears continuous, showing all colors from red, through orange, yellow, green, and blue, to violet.

With the lights on, display the white porcelain dish to the audience, showing that the outer surface is clean. Hold the bottom of the dish in the white region near the top of the flame for a few seconds. Show the audience the black spot of soot that has deposited on the bottom of the dish.
Extinguish the candle.

PROCEDURE C

Preparation and Presentation

This demonstration needs to be done in a room that can be darkened as completely as possible, including closing blinds, shutting doors, and so on.

When using a transmission diffraction grating, tape the grating in front of the camera lens. Focus the camera on the flame and then turn the camera slightly to the side until the spectrum is displayed on the screen. Darken the room. The spectrum appears continuous, showing all colors from red, through orange, yellow, green, and blue, to violet.

When using a compact disc, display the spectrum of the candle flame via video projection by mounting the disc behind and somewhat to the side of the flame (from the perspective of the audience). Mask the flame from the audience, so only its reflection from the disc can be seen. Focus the camera on the reflection from the disc and tip the disc sideways a bit to display the spectrum on the screen. Darken the room. The spectrum appears continuous, showing all colors from red through orange, yellow, green, and blue to violet.

With the lights on, display the white porcelain dish to the audience, showing that the outer surface is clean. Hold the bottom of the dish in the white region near the top of the flame for a few seconds. Show the audience the black spot of soot that has deposited on the bottom of the dish.
Extinguish the candle.

HAZARDS

Be careful with the open flame and be sure no flammable liquids or other materials are nearby, and extinguish the flame as soon as the audience has had time to explore the spectrum.

DISPOSAL

Clean the porcelain dish. Materials may be saved and used in future presentations.

DISCUSSION

Until only a little more than a century ago, all artificial light was produced by fire, which is the result of a chemical reaction in which a fuel combines with oxygen in the air. Numerous types of fuels have been used, including wood and vegetable oils. Lamps that use liquid fuels are particularly convenient for producing light because a liquid can be used with a wick. Via capillary action, the wick delivers a steady, controlled flow of fuel to a flame, thereby producing a light of constant brightness. A disadvantage of liquid fuel is that it can be spilled, and a spilled fuel can lead to an uncontrolled fire.

The candle is a technological marvel that exploits the advantages of a liquid fuel but dispenses with its disadvantages. A candle has two components, wax and a wick. Wax is a solid fuel that melts at a low temperature, and because it is normally a solid, there is no danger of spilling fuel. The wax used in candles has several different sources, and its composition depends on its source. Most candles are made from either beeswax or paraffin wax.

Beeswax is a mixture of hydrocarbons, alcohols, fatty acids, and esters [1], but 95% of it is in the form of chains of methylene, CH2, groups [2]. Paraffin wax is a petroleum product consisting of a mixture of predominantly straight-chain hydrocarbons, also consisting of CH2 groups, with more than 20 carbon atoms.
How a candle works was famously explained by Michael Faraday in a series of six lectures to an audience of young people at the Royal Institution of Great Britain in London during the Christmas holidays of 1860–61 and published as the book The Chemical History of a Candle in 1861 [3]. When the tip of the wick is ignited, the heat of the flame melts a small amount of the wax, and the liquid wax is carried by capillary action up the wick into the flame. Beeswax melts at about 60°C, and paraffin wax melts in the range 52 to 57°C. The molten wax enters the flame, is vaporized by the heat, and burns. The heat from the burning wax melts more wax, which sustains the flame.

The flame of a candle is not uniform. At least two regions can be easily observed: a pale bluish region near the bottom of the flame and a bright yellowish region from the center to the top. Less easily visible is a dark region at the center near the wick. When molten wax moves up the wick, some of it is vaporized near the bottom of the flame. This region is rich in oxygen from the air.

The wax vapor and oxygen react to give off the same blue glow as any gas burning in ample oxygen. The glow is produced mainly by excited C2 and CH molecules in the gas (see the section Flames in the introduction, page 61) and is identical to the blue glow when natural gas burns in a kitchen stove or propane burns in a gas grill. The blue region in the candle flame extends up around the outside of the entire flame, where there is ample oxygen, but, the blue is obscured by the bright light from the upper regions of the flame.

Some of the liquid wax moves up the wick, above the bottom of the flame, and vaporizes near the top of the wick. Here, there is little oxygen, because most has been used up below. As the wax vapor moves up and away from the wick, it is heated to a high temperature by the flame, but because there is no oxygen with which it can combine, the molecules in the vapor decompose in the heat, forming very tiny particles of solid carbon, as well as hydrogen gas. If a cool object, such as a porcelain dish, is held in the center of the flame, some of these carbon particles can be collected, producing a black spot of carbon soot on the object held in the flame. In addition to bits of graphite, this collected soot also contains small amounts of buckminsterfullerenes, ball-shaped molecules of C60, C70, and their fragments. (See the section Flames in the introduction, page 61.) The tiny particles of carbon and other products of the thermal decomposition of wax vapor drift upward and outward from the wick, where they encounter more oxygen, and there they undergo combustion.

The tiny particles of carbon that form in the yellow region of a candle flame are very hot, about 1000°C [4], and anything that hot emits visible light. The light emitted by the carbon particles gives the flame its bright yellowish glow, but the hot particles of carbon in the candle flame actually emit light of various colors. This emission can be observed by separating the colors using a diffraction grating, as shown in this demonstration. The spectrum of light emitted by a candle flame appears to be a continuous spectrum, with every wavelength from deep red to pale violet represented, although with different intensities. The light is brightest in the orange and yellow regions of the spectrum, and very dim, at best, in the blue and violet regions.

All matter constantly emits electromagnetic radiation. Objects at room temperature emit radiation mainly in the infrared region of the spectrum. As the temperature of an object increases, it emits more energy, and the wavelengths of the emitted radiation become shorter. As the object gets hotter, more and more of the emitted radiation is visible, and the object begins to glow. First, at around 700°C, it glows red, the color corresponding to the lowest energy of visible light. As the temperature increases, visible light of higher energy is emitted, from orange to yellow, to green, to blue, and to violet. When an object is hot enough to be emitting all visible wavelengths, its glow appears white, and we can say that it is “white hot.” The emission of visible light by a hot object is called incandescence.

The electromagnetic radiation emitted by objects as a result of their temperature is called black-body radiation because it does not include reflected radiation; a black object is black because it does not reflect light. (See the section Black-body Radiation in the introduction, page 23.) The distribution of the wavelengths in black-body radiation is determined by temperature. Thermometers that display the temperature of an object at a distance detect black-body radiation and convert it to a temperature reading. Physicians commonly use this kind of thermometer to measure an infant’s body temperature by placing a probe into the child’s ear. The thermometer detects the radiation from the ear drum and displays the corresponding temperature.

If the emission from the flame of a candle were purely black-body radiation, the emission could be used to determine its temperature. Such a determination indicates the temperature to be about 1550°C [5]. Emission from a solid incandescent object at 1550°C looks yellow to an observer. However, measurements of the flame temperature using other methods show that the flame is at most 1400°C in the blue region, and in other regions, considerably less than that. Hence, the spectrum of radiation given off by a candle flame is not purely black-body radiation; it is, instead, the combination of numerous line spectra emitted by many different tiny particles of carbon.

The carbon particles in the flame are small enough to behave as molecules having distinct excited states, which are attained through the absorption of thermal energy in the flame. As the particles return to their ground states, they emit a narrow band of radiation. Because each particle has numerous excited states and there are numerous different particles with different excited states, the light emitted contains many closely spaced wavelengths, which give the appearance of a continuous spectrum.

The use of a video projection system to display the spectrum in this demonstration presents an opportunity to point out the difference between physical and physiological color perception. (See the section Color Perception in the introduction, page 4.) Consider the color yellow observed directly in the candle flame and in the projected spectrum. The color perceived from the flame is the physical color resulting from the action of wavelengths in the yellow region of the spectrum on the pigments in the retina of the observers’ eyes. The yellow color perceived in the video-projected spectrum, on the other hand, is produced by a combination of red, green, and blue emissions in the projection system. This combination acting on the pigments in the observers’ retinas provides the physiological perception of yellow, even though there are no yellow wavelengths in the projected light.

REFERENCES

1. A.P. Tulloch and L. L. Hoffman, “Canadian Beeswax: Analytical Values and Composition of Hydrocarbons, Free Acids, and Long Chain Esters,” J. Am. Oil Chemists’ Soc.,49, 696–699 (1972).
2. T. Kameda, “Molecular Structure of Crude Beeswax Studied by Solid-State 13C NMR,” J. Insect Sci., 4, 29–33 (2004).
3. M. Faraday, “A Course of Six Lectures on the Chemical History of a Candle: To WhichIs Added a Lecture on Platinum,” published as The Chemical History of a Candle,W. Crookes, ed., Griffin, Bohn, and Co.: London (1861). The Chemical History of aCandle is no longer copyrighted in the United States and many versions are availableelectronically.
4. A. G. Gaydon and H. G. Wolfhard, Flames: Their Structure, Radiation and Temperature, 4th ed., p. 155, Chapman and Hall: London (1979).
5. G. W. Stewart, “The Temperatures and Spectral Energy Curves of Luminous Flames,” Phys. Rev. (Series I), 15, 306–315 (1902).