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Fig. 9.8 Variation of solar radiation over wavelengths.

The output of the sun's energy also varies over wavelengths (Fig. 9.8). To understand the balance of radiation we need to see what curve represents the energy given off by the earth. The earth is absorbing, as we already discussed, quite a percentage of the solar radiation that strikes it. The albedo, or amount reflected by the earth, is only about 30% of that reaching the earth. The earth is giving off infrared energy according to the fourth power of the temperature of the earth. The peak of this energy given off is at 10 µm. The sun's peak is somewhere around 0.5 µm wavelength (Fig. 9.9). Thus, the sun is a shortwave emitter. The earth absorbs this shortwave and re-radiates as longwave radiation.

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Fig. 9.9 Wavelength of peak radiation for the sun and the Earth.

An interesting thing, discovered many years ago, was that the amount of energy under the long-wave curve, increases as something gets hotter (Fig. 9.10). That's easy to discover. As you build a bigger fire in a stove and it gets hotter, you can feel more energy striking you. Obviously it gives off more energy. But the peak is not simply higher; the peak moves to a shorter wavelength. Something at room temperature, 68° F (20 °C), has its peak at 10 µm. If it is heated, the peak also moves. The area under the curve expands. When it is heated more, the peak moves more, until you've heated this thing to around 6,000 K (3315° C). At this temperature the peak moves to about 0.5 µm in the visible wavelength. Something which has been heated until it became "red hot" can be seen.

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Fig. 9.10 Shift of peak wavelength of emission at hotter temperatures.

An example would be a light bulb with a brightness control on it. Adjusting the brightness control on the light switches the color from a nice white light, to yellowish light, and just before the bulb goes out, a red. Looking at the filament of the light bulb, you would see it glowing a deep red just before it goes out and disappears.

Clearly there are two ways that we could utilize the characteristics of this curve to determine the temperature of something. (1) Measure the area under the curve and say that represents a certain amount of energy that is given off by the object. If the object is hotter, there will be a lot more energy under the curve. (2) Measure where the peak of the energy emitted by an object is The first way that temperature was measured remotely was by the area under the curve. When feeling warmth from the stove, you are feeling the energy under the curve. When the stove is so hot it begins to be red hot, then you're seeing the peak. You can detect both characteristics as things approach red hot.