21) what wavelength does the earth emit most of its radiation at, and what is the name of that radiation region?

Longwave terrestrial radiation at the night time, in the of infrared rays.

Explanation:

As the short wave solar radiation or the visible light containing lots of energy the longwave radiation of the earth contains infrared that has less energy. As the solar radiation that enters our atmosphere in the form of UV rays, and sun emits these short waves as they are extremely hot and hence have lots of energy to give off. As parts of incoming solar are absorbed by the earth's surface features and rest of is reflected into the atmosphere by cloud cover and the energy that falls on the ground is reradiated back to the sun as the earth is cooler at night and less energy is given off. Hence the atmosphere tires to balance the incoming energy from the sun in the form of a heat budget and a greenhouse effect.