 We can see what happens as we change the infrared absorptivity and desimissivity for cases in which there are two sources of radiation, a solid object like Earth and the atmosphere, which can both absorb and emit. Look at Schwarzschild's equation. The IDS is a change in radiance over distance. Kappa A is the atmospheric absorption coefficient in units of meters to the minus one. P sub E is the Planck distribution function spectral radiance in that same direction. And I is the radiance of the source. By the way, the atmosphere's temperature is low enough that P sub E is significant in the infrared, but not in the visible, where it is very small. So in the visible, Schwarzschild's equation becomes the same as P's log absorption because P sub E is essentially zero. Not so in the infrared. Note that if P sub E equals I, then P sub IDS equals zero. That is, the radiation does not change with distance. Let's look at a system where there is a radiation source at a single wavelength, and in between the source and the observer is an atmosphere. We have seen that the amount of absorption and thus emissivity depends strongly on the wavelength of the infrared radiation. So let's look at some different wavelengths effectively. Let's start with a case in which the atmosphere does not absorb at all. Thus, the absorptivity and emissivity are zero, and all the observer sees is a source but nothing from the atmosphere. For a second case where absorptivity is very small, there is some absorption of the source radiation, but there is also some emissions from the atmosphere itself. For the third case of stronger absorptivity, the absorption is stronger so that the source is harder to see, but the atmosphere is now emitting more. Finally, if the absorptivity and emissivity equal one, then all the radiation from the source is absorbed and all the observer sees is the atmosphere nearer.