Radiation
You can feel heat transfer from a fire or the Sun. Yet the space between Earth and the Sun is largely empty, without any possibility of heat transfer by convection or conduction. Similarly, you can tell that an oven is hot without touching it or looking inside—it just warms you as you walk by.
In these examples, heat is transferred by radiation. The hot body emits electromagnetic waves that are absorbed by our skin, and no medium is required for them to propagate. We use different names for electromagnetic waves of different wavelengths: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays .
Radiation from a Fire
Most of the heat transfer from this fire to the observers is through infrared radiation. The visible light, although dramatic, transfers relatively little thermal energy. Convection transfers energy away from the observers as hot air rises, while conduction is negligibly slow here. Skin is very sensitive to infrared radiation so that you can sense the presence of a fire without looking at it directly.
The energy of electromagnetic radiation depends on its wavelength (color) and varies over a wide range; a smaller wavelength (or higher frequency) corresponds to a higher energy. We can write this as:
where E is the energy, f is the frequency, λ is the wavelength, and h is a constant.
Because more heat is radiated at higher temperatures, a temperature change is accompanied by a color change. For example, an electrical element on a stove glows from red to orange, while the higher-temperature steel in a blast furnace glows from yellow to white. The radiation you feel is mostly infrared, which is lower in temperature still.
The radiated energy depends on its intensity, which is represented by the height of the distribution .
Radiation Spectrum
(a) A graph of the spectra of electromagnetic waves emitted from an ideal radiator at three different temperatures. The intensity or rate of radiation emission increases dramatically with temperature, and the spectrum shifts toward the visible and ultraviolet parts of the spectrum. The shaded portion denotes the visible part of the spectrum. It is apparent that the shift toward the ultraviolet with temperature makes the visible appearance shift from red to white to blue as temperature increases. (b) Note the variations in color corresponding to variations in flame temperature.
Heat Transfer
All objects absorb and emit electromagnetic radiation. The rate of heat transfer by radiation is largely determined by the color of the object. Black is the most effective, and white the least. People living in hot climates generally avoid wearing black clothing, for instance. Similarly, black asphalt in a parking lot will be hotter than the adjacent gray sidewalk on a summer day, because black absorbs better than gray. The reverse is also true—black radiates better than gray. Thus, on a clear summer night the asphalt will be colder than the gray sidewalk because black radiates energy more rapidly than gray.
An ideal radiator, often called a blackbody, is the same color as an ideal absorber, and captures all the radiation that falls on it. In contrast, white is a poor absorber and also a poor radiator. A white object reflects all radiation, like a mirror. (A perfect, polished white surface is mirror-like in appearance, and a crushed mirror looks white. )
There is a clever relation between the temperature of an ideal radiator and the wavelength at which it emits the most radiation. It is called Wien's displacement law and is given by:
where b is a constant equal to 2.9×10-3 m⋅K.
Gray objects have a uniform ability to absorb all parts of the electromagnetic spectrum. Colored objects behave in similar but more complex ways, which gives them a particular color in the visible range and may make them special in other ranges of the nonvisible spectrum. Take, for example, the strong absorption of infrared radiation by the skin, which allows us to be very sensitive to it .
Good and Poor Radiators
A black object is a good absorber and a good radiator, while a white (or silver) object is a poor absorber and a poor radiator. It is as if radiation from the inside is reflected back into the silver object, whereas radiation from the inside of the black object is "absorbed" when it hits the surface and finds itself on the outside and is strongly emitted.
The rate of heat transfer by emitted radiation is determined by the Stefan-Boltzmann law of radiation:
where =5.67×10−8 J s-1⋅m-2⋅K-4 is the Stefan-Boltzmann constant, A is the surface area of the object, and T is its absolute temperature in kelvin. The symbol e stands for the emissivity of the object, which is a measure of how well it radiates. An ideal jet-black (or blackbody) radiator has e=1, whereas a perfect reflector has e=0. Real objects fall between these two values. For example, tungsten light bulb filaments have an e of about 0.5, and carbon black (a material used in printer toner), has the (greatest known) emissivity of about 0.99.
The radiation rate is directly proportional to the fourth power of the absolute temperature—a remarkably strong temperature dependence. Furthermore, the radiated heat is proportional to the surface area of the object. If you knock apart the coals of a fire, there is a noticeable increase in radiation due to an increase in radiating surface area.
Net Rate of Heat Transfer
The net rate of heat transfer by radiation (absorption minus emission) is related to both the temperature of the object and that of its surroundings. Assuming that an object with a temperature T1 is surrounded by an environment with uniform temperature T2, the net rate of heat transfer by radiation is:
where e is the emissivity of the object alone. In other words, it does not matter whether the surroundings are white, gray, or black; the balance of radiation into and out of the object depends on how well it emits and absorbs radiation. When T2>T1, the quantity Qnet/t is positive; that is, the net heat transfer is from hotter objects to colder objects.