Blackbody radiation is not confined to a particular wavelength or even waveband—a range of wavelengths. The amount of energy a blackbody radiates is proportional to its temperature, and its temperature determines the wavelength at which it radiates most intensely, but to either side of this peak the radiation tails off toward longer and shorter wavelengths.
Visible light forms only a part of the electromagnetic spectrum, but it is the region at which the Sun radiates most intensely, in the 0.4-0.7 pm waveband. Our eyes are sensitive to radiation in this waveband, but they cannot detect radiation at wavelengths shorter than 0.380 pm (violet) or longer than 0.765 pm (red). The Sun radiates across a much wider spectrum than this, however, and therefore most of its radiation is invisible (see the sidebar).
Earth also acts as a blackbody. Its surface is much cooler than that of the Sun and so it emits much less energy and radiates at much longer wavelengths. The spectrum of terrestrial radiation peaks at 9-15 pm. When the spectra of the Sun and Earth are shown graphically, as in the illustration, it is clear that their radiation peaks are widely separated. The peak of the solar spectrum lies in the waveband of visible light—in fact, between yellow and green light, at 0.577-0.597 pm. Earth's spectrum peaks in the infrared waveband—called "infrared" because it lies beyond the wavelength of red light (0.765 pm).
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.