All objects continually emit and absorb electromagnetic radiation. The emitted radiation is given the special name blackbody radiation.
The spectrum of the radiation emitted by an object is continuous. The temperature of the object determines the wavelength at which the spectrum is maximum and the total energy output per unit time.
4.3.1 Introduction
4.3.2 Examples of Blackbody
Radiation
4.3.3 The Spectrum of Blackbody
Radiation
4.3.4 Blackbody Emissions and
Temperature
4.3.5 Absorptivity and Emissivity
4.3.7 Key Points about Radiant Energy and
Blackbody Radiation
All material objects emit electromagnetic radiation; the distribution of photon energies and fluxes emitted depend primarily on the object's temperature. This phenomenon is known as blackbody radiation. Because the amount of radiation, and its spectrum depends on the temperature, it is sometimes called thermal radiation, or heat radiation.
The object in question can be large (stars and planets), small (single molecules), solid, liquid, or gaseous. Blackbody radiation is a familiar phenomenon: When the temperature of an object (such as a piece of metal) is increased, it begins to glow reddish-orange, and, as the temperature is further increased, its glow becomes progressively whiter. As the temperature is further increased, the glow takes on a bluish cast, however, at such high temperature, the glow is usually so intense that it is painful to look at, and even harmful to the eyes (which is why welders use dark goggles when working).
Even when an object is cool, and we do not see a glow at all, the object is constantly emitting radiation that is mostly in the infrared region. Night vision equipment detects this infrared radiation, and electronically converts the image detected in the infrared to a visible image.
Blackbody radiation is continually removing energy from an object, thereby causing it to cool. This is the reason that the Earth's surface cools at night. Why doesn't an object keep cooling and cooling, eventually reaching the absolute zero of temperature? The reason is that at the same time the object is losing energy to outgoing blackbody radiation, it is bathed in the blackbody radiation emitted by everything else in its surroundings, and it absorbs some of this radiation, replacing some of the energy that is being lost. Electromagnetic radiation is continually being "exchanged" among objects. Objects that are warmer emit more energetic radiation than those that are cooler and so they cool faster. Therefore, in the absence of an external heat source, all objects in a confined space will eventually reach the same temperature. That is, they will reach thermal equilibrium. Even after thermal equilibrium is reached, the objects still exchange radiation with each other, but now the objects are absorbing and emitting energy in the form of radiation at exactly the same rate, so no net heat exchange takes place.
The blackbody radiation of the Sun, the Earth, the Earth's atmosphere, and clouds play an important role in the Earth's climate. Most of the Sun's radiation is blackbody radiation radiated from the Sun's surface, or photosphere, whose temperature is about 5700 K. The Earth's surface is warmed by absorbing this light. At the same time it is absorbing energy, the Earth's surface is emitting its own blackbody radiation. At night, when the surface is no longer being illuminated by the Sun, it is still radiating its own blackbody radiation, the Earth's surface to cool. Some of that radiation is absorbed by the atmosphere. The atmosphere also emits blackbody radiation, some of which is absorbed by the Earth's surface. The nighttime temperature depends upon the relative rates of absorption and emission by the Earth and atmosphere. If the atmosphere is made more efficient at absorbing radiation, it will trap more of the Earth's radiation, and re-radiate more of it downward, making the Earth's surface warmer on average. This is known as the greenhouse effect.
4.3.3 The Spectrum of Blackbody Radiation
Now let's take a more quantitative look at the absorption and emission of electromagnetic radiation by any macroscopic object. An ideal blackbody is a hypothetical object that absorbs all radiation incident on its surface. (Hence the name blackbody, since something that doesn't reflect any light will appear black). Physical theory predicts the spectral irradiance emitted by an ideal blackbody at a certain temperature T. The figure below shows the theoretical spectrum of an ideal blackbody at a number of different temperatures. Notice that in this figure, each major tick-mark represents a factor of 10 difference from the adjacent tick-mark. That is, both the wavelength and irradiance scales are logarithmic.
Spectrum of radiation emitted by an ideal blackbody radiator at temperatures T=6000K, 5000K, 4000K, 3000K, 2000K, and 1000K. The Sun's photosphere is a little less than 6000K. The effective temperature of the Earth's surface is a little less than 300K.
One hallmark feature of blackbody radiation is that it has a
continuous spectrum. That is, the irradiance curve is smooth.
It doesn't have any wiggles in it, and it doesn't abruptly go to zero
at some wavelength.
Nonideal blackbodies emit less radiation any given wavelength than an
ideal blackbody would.
Ideal blackbodies do
not actually exist. However the radiant energy emitted by
many objects, such as the
Earth and Sun, can be closely matched by the emission of a blackbody
at the same temperature.
The amount of emission of a blackbody
at each wavelength depends only on the absolute temperature (in
kelvin) of the blackbody. Remember that heat is a form of energy so
it follows that if a blackbody is heated by absorption of more
energetic photons (or through other processes such as conduction) it
will emit more energetic photons.
4.3.4 Blackbody Emissions and Temperature
We have already seen that blackbodies emit electromagnetic radiation
at all wavelengths (energies) in the spectrum and that the intensity
of the emitted radiation varies with wavelength, depending on the
temperature of the object. The figure above shows the spectrum of
electromagnetic radiation emitted by blakcbody radiators, as a
function of wavelength for different temperatures.
When photons are emitted over a continuous range of the spectrum as we see from these curves, we call it continuum radiation. What happens to the peak in each curve as you go to higher temperatures? You can see that the peak in each curve shifts to shorter wavelengths (higher energies) with increasing temperature.
4.3.5 Absorptivity and Emissivity
Although an ideal blackbody is hypothetical, objects are often
identified by how their radiative properties compare to those of a
blackbody at the same temperature. Two important related properties
of objects are their absorptivity and emissivity. The absorptivity,
a, of an object is defined as the fraction of incident radiation that
is absorbed by an object at a specific wavelength . The emissivity,
e, of a material is the fraction of radiation emitted at a specific
wavelength compared to that emitted by a blackbody at the same
temperature. As we know from before, an ideal blackbody absorbs all
incident radiation and emits the maximum amount possible at each
wavelength. Therefore, the absorptivity and emissivity of an ideal
blackbody at any wavelength are equal to 1. For real objects these
values may vary from 0 to 1. Kirchoff's law states that for
any object:
a = e
According to this, an object that is a strong absorber at a particular wavelength is also a strong emitter at that wavelength, and an object that absorbs weakly at a particular wavelength will emit weakly at that wavelength. Whether or not an object is a strong or weak emitter at a given wavelength depends on the characteristics of the material. The emissivities of various components in the atmosphere play a vital role in determining the temperature structure of the atmosphere.
4.3.7 Key Points about Radiant Energy and Blackbody Radiation