![]() ![]() The rate of heat transfer by emitted radiation is determined by the Stefan-Boltzmann law of radiation: 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. 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. Take, for example, the strong absorption of infrared radiation by the skin, which allows us to be very sensitive to it.įigure 4. 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. Gray objects have a uniform ability to absorb all parts of the electromagnetic spectrum. (A perfect, polished white surface is mirror-like in appearance, and a crushed mirror looks white.) A white object reflects all radiation, like a mirror. In contrast, white is a poor absorber and is also a poor radiator. An ideal radiator is the same color as an ideal absorber, and captures all the radiation that falls on it. Thus, on a clear summer night, the asphalt will be colder than the gray sidewalk, because black radiates the energy more rapidly than gray. The reverse is also true-black radiates better than gray. Similarly, black asphalt in a parking lot will be hotter than adjacent gray sidewalk on a summer day, because black absorbs better than gray. People living in hot climates generally avoid wearing black clothing, for instance (see Take-Home Experiment: Temperature in the Sun). Black is the most effective, and white is the least effective. The rate of heat transfer by radiation is largely determined by the color of the object. The thermal conductivities of the pavements are the same.Īll objects absorb and emit electromagnetic radiation. 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 radiated energy depends on its intensity, which is represented in Figure 2 by the height of the distribution.Įlectromagnetic Waves explains more about the electromagnetic spectrum and Introduction to Quantum Physics discusses how the decrease in wavelength corresponds to an increase in energy.įigure 3. The radiation you feel is mostly infrared, which corresponds to a lower temperature than that of the electrical element and the steel. Take, for example, an electrical element on a stove, which glows from red to orange, while the higher-temperature steel in a blast furnace glows from yellow to white. Because more heat is radiated at higher temperatures, a temperature change is accompanied by a color change. The energy of electromagnetic radiation depends on the wavelength (color) and varies over a wide range: a smaller wavelength (or higher frequency) corresponds to a higher energy. Skin is very sensitive to infrared radiation, so that you can sense the presence of a fire without looking at it directly. Convection transfers energy away from the observers as hot air rises, while conduction is negligibly slow here. ![]() The visible light, although dramatic, transfers relatively little thermal energy. Most of the heat transfer from this fire to the observers is through infrared radiation.
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