<P> When light passes through Earth's atmosphere, photons interact with it through scattering . If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun . Indirect radiation is light that has been scattered in the atmosphere . For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered . As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths . This is why the sky looks blue; you are seeing scattered blue light . This is also why sunsets are red . Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye . Much of the blue light has been scattered out, leaving the red light in a sunset . </P> <P> Different molecules absorb different wavelengths of radiation . For example, O and O absorb almost all wavelengths shorter than 300 nanometers . Water (H O) absorbs many wavelengths above 700 nm . When a molecule absorbs a photon, it increases the energy of the molecule . This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below . </P> <P> The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light . The optical window runs from around 300 nm (ultraviolet - C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400--700 nm and continues to the infrared to around 1100 nm . There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths . For example, the radio window runs from about one centimeter to about eleven - meter waves . </P> <P> Emission is the opposite of absorption, it is when an object emits radiation . Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths . Colder objects emit less radiation, with longer wavelengths . For example, the Sun is approximately 6,000 K (5,730 ° C; 10,340 ° F), its radiation peaks near 500 nm, and is visible to the human eye . Earth is approximately 290 K (17 ° C; 62 ° F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans . </P>

What is the ratio of the number of oxygen to nitrogen molecules in earth's lower atmosphere