Here's why:
* Earth's temperature: Earth's surface is relatively cool compared to the sun.
* Wien's Displacement Law: This law states that the peak wavelength of radiation emitted by an object is inversely proportional to its temperature. Since Earth is cooler, its peak radiation falls in the longer wavelengths of the infrared.
While Earth does emit some radiation in other wavelengths, the majority of its energy is radiated as infrared radiation. This is why we use infrared cameras to detect heat, as they are sensitive to the radiation emitted by warm objects like Earth.
