Clive M. Countryman

“ Pure white is seen when all visible wavelengths are being reflected, and pure black is seen when all visible wavelengths are being absorbed and none reflected. Actually, we cannot see a truly black object, only the blackness from lack of reflected light in contrast to the light reflected from other substances. ”

“ Radiant energy travels outward in all directions from the emitting substance until it encounters something capable of absorbing it. The absorbed radiation increases the molecular activity in the substance, thereby increasing its temperature and the amount of heat it contains—this is why we frequently sense radiation as heat. The heated substance radiates energy too, and this energy can be absorbed by other substances and converted to heat. ”

“ Dark-colored materials are usually better emitters and absorbers of radiation than are light-colored materials, and opaque materials are better than transparent ones. A substance capable of emitting and absorbing all thermal radiation is called a black body.
Not all thermal radiation reaching a substance is absorbed. Part of the radiation may be transmitted unchanged through the substance, and some may be reflected. Reflection does not change the wavelength of the radiation, only its direction of travel. Only absorbed radiation is converted to heat. ”

“ Radiation intensity changes with distance and angle The intensity of radiation received by an object depends not only on the temperature of the radiation source, but also on the object's distance from the source and the angle at which the radiation strikes. For a point source of radiation, the radiation intensity decreases inversely as the square of the distance. This means the radiation intensity 20 feet from the source is only one-fourth that at 10 feet. Intensity decreases so rapidly because the radiant energy waves travel only along straight paths. ”

“ Radiation intensity and wavelength change with temperature Substances at ordinary atmospheric temperatures emit thermal radiation that is mostly in the long wave or infrared range. As the temperature of a substance becomes higher, the total amount of radiation becomes greater. But the amount of radiation in the shorter wavelengths increases more rapidly with rising temperature than does that in the longer wavelengths. Therefore, as the temperature of a substance increases, the wavelength of maximum radiation intensity shifts more and more toward the shorter wavelength part of the spectrum. ”

“ All substances radiate energy when their temperature is above absolute zero. Radiant energy is not the same as thermal energy or heat, but radiant energy can be converted to heat and heat can be converted to radiant energy. Heat transfer by radiation is accomplished through this interconvertibility of radiant energy and heat. Radiation is the only means of heat transfer that does not require an intervening substance between the heat source and the receiving substance.
Radiant energy varies greatly in wavelength. ”

“ The term black body is somewhat misleading, because some substances that are not black are almost perfect radiators and absorbers of radiation. And a perfect black body, if it is hot enough to radiate in the visible light range, does not appear black. Thick flames in a wildland fire, for example, can come as close to emitting thermal radiation as a black body. ”

“ All of the radiant energy arranged in the order of wavelength forms the electromagnetic spectrum of radiant energy.
Most radiant energy is invisible, and its wavelength and often its presence can be detected only with special instrumentation. Visible light occupies a narrow band near the middle of the spectrum, and its wavelengths can be readily distinguished because light waves of differing lengths are visible as different colors. Radiation from the sun includes all visible wavelengths. ”

“ Radiation Wildland fire is dependent on heat transfer. For a fire to start, heat must be transferred from a firebrand to the fuel. If the fire is to burn and grow, heat transfer to the unburned fuel around the fire must continue. The way a fire burns and behaves is closely related to the manner and rate of heat transfer. The speed with which fire spreads, for example, depends greatly on how quickly sufficient heat for ignition can be transferred to the unburned fuel. ”

“ Radiation strikes the receiving object from all of the points—in greatest intensity from points perpendicular and closest to the object, and in lesser intensity from the other points. Because so many points are producing radiant energy, the decrease in intensity with distance from a flame source is less than that from a line source and much less than that from a point source. The number of points from which radiation can be received also increases rapidly with the area of the radiating surface. ”

“ All substances at temperatures above absolute zero (-469° F) , produce radiant energy. On earth, absolute zero can be approached only in the laboratory and with special apparatus. For all practical purposes, then, we can consider that all materials we ordinarily deal with are radiating energy—even cold ones. ”

“ Dead wildland fuels usually reflect less than 30 percent. The tops of clouds reflect a large part of the solar radiation—40 to 80 percent. Thus, the diminished sunlight on a cloudy day is not due so much to absorption by the clouds as to reflection back to space.
Substances tend to attain a common temperature We have seen that all substances above a temperature of absolute zero emit radiation. If two objects with different temperatures are placed in an evacuated and perfectly insulated enclosure so no heat can be lost or external heat gained, they can radiate energy only to each other. ”

“ About 36 percent of the solar radiation received on the earth, taken as a whole, is reflected, but this percentage varies greatly with the kind of surface. Clean, fresh snow reflects about 80 percent of the sun's radiation, dry sand about 30 percent, and dark coniferous forests about 5 percent. ”

“ This is why it is better to wear white rather than dark clothing on a hot day, and why a dark-colored car parked in the sun is noticeably warmer to the touch than a white car parked next to it. In some of the early nuclear tests in the South Pacific, dark-colored birds were killed by thermal radiation from the nuclear bomb, whereas white birds survived because they absorbed less of the thermal radiation. ”