Selective absorption of the atmosphereThe gasses that comprise our atmosphere are referred to as selective absorbers. That is, each gas absorbs only particular wavelengths of light. Why? Electrons orbit the nucleus of an atom at fixed orbital distances called orbital shells. The orbital shell for each atom is different and discrete. That is, for a given atom like hydrogen, its electrons can only orbit at particular distances and are different than those for atoms of neon.
Each orbital shell is associated with a given energy level; the greater the distance from the nucleus the greater the energy level. Electrons jump to a higher shell when excited by the absorption of energy. The photon must have the exact amount of energy to move the electron from, say, shell one to shell two. If the photon doesn't have enough energy to move the electron to shell two, it cannot move the electron half way between shell one and two. The atom does not stay in this excited, unstable state for very long. Energy is given off and the electron returns to a stable state or its "ground state" (lowest energy level or orbital distance). Recall that the amount of energy carried by a photon depends on the wavelength. Thus the atoms that comprise a gas can only absorb, or emit, particular wavelengths of energy (i.e. photons of energy). We can see this selective absorption by examining Figure ER.7 below. The graph shows very little absorption for the atmosphere as a whole in the shortwave end of the spectrum, especially in the visible light band (the band of maximum emission for the Sun). The atmosphere absorbs far better in the longwave end of the electromagnetic spectrum which is the region of maximum emission (10µm) for the Earth.
Figure ER.6 Absorptivity of the atmosphere and selected gasses. |
