Box 20-1 Blackbody Radiation and the Greenhouse Effect

When an object is heated, it emits radiation—it glows. Even at room temperature, objects radiate at infrared frequencies. Imagine a hollow sphere whose inside surface is perfectly black. That is, the surface absorbs all radiation striking it. If the sphere is at constant temperature, it must emit as much radiation as it absorbs. If a small hole were made in the wall, we would observe that the escaping radiation has a continuous spectral distribution. The object is called a blackbody, and the radiation is called blackbody radiation. Emission from real objects such as the tungsten filament of a light bulb resembles that from an ideal blackbody. 

The power per unit area radiating from the surface of an object is called the exitance (or emittance), M, and is given by 

The blackbody emission spectrum at right changes with temperature, as shown in the graph. Near 300 K, maximum emission occurs at infrared wavelengths. The outer region of the sun behaves like a blackbody with a temperature near 5 800 K, emitting mainly visible light. 

Of the solar flux of 1 368 W/m2 reaching the upper atmosphere, 23% is absorbed by the atmosphere and 25% is reflected back into space. Earth absorbs 48% of the solar flux and reflects 4%. Radiation reaching Earth should be just enough to keep the surface temperature at 254 K, which would not support life as we know it. Why does the average temperature of Earth s surface stay at a comfortable 287 K  

Balance between energy reaching Earth from the sun and ei .  

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