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The figure shows a close up of two sets of ice cubes. The ice on the left is on a light-colored pavement and is relatively unmelted. The ice on the right is on a darker pavement and is noticeably more melted.
This illustration shows that the darker pavement is hotter than the lighter pavement (much more of the ice on the right has melted), although both have been in the sunlight for the same time. The thermal conductivities of the pavements are the same.

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.

In the figure two black and two silver polished blocks are shown. Radiant energy is incident on the first black block. Most of the energy is absorbed and only a small amount is shown as reflected. On the second black block more of the energy from inside the block is emitted than is retained. On the first silver polished block the incident energy is mostly reflected and only a small portion is absorbed. On the second silver polished block the energy from inside is mostly retained and only a small amount of energy is emitted.
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    :

Q t = σ e A T 4 , size 12{ { {Q} over {A} } =σ`e`A`T rSup { size 8{4} } } {}

where σ = 5 . 67 × 10 8 J/s 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 black body) radiator has e = 1 , whereas a perfect reflector has e = 0 . Real objects fall between these two values. Take, for example, tungsten light bulb filaments which have an e of about 0 . 5 , and carbon black (a material used in printer toner), which 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.

A thermograph of part of a building is shown. The temperature variations are shown in different colors. The region near the windows is more yellowish green, with reddish frames, showing that the windows are major regions of radiative heat transfer.
A thermograph of part of a building shows temperature variations, indicating where heat transfer to the outside is most severe. Windows are a major region of heat transfer to the outside of homes. (credit: U.S. Army)

Skin is a remarkably good absorber and emitter of infrared radiation, having an emissivity of 0.97 in the infrared spectrum. Thus, we are all nearly (jet) black in the infrared, in spite of the obvious variations in skin color. This high infrared emissivity is why we can so easily feel radiation on our skin. It is also the basis for the use of night scopes used by law enforcement and the military to detect human beings. Even small temperature variations can be detected because of the T 4 size 12{T rSup { size 8{4} } } {} dependence. Images, called thermographs , can be used medically to detect regions of abnormally high temperature in the body, perhaps indicative of disease. Similar techniques can be used to detect heat leaks in homes [link] , optimize performance of blast furnaces, improve comfort levels in work environments, and even remotely map the Earth’s temperature profile.

Practice Key Terms 5

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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