The atmosphere is the mixture of gas molecules and other materials surrounding the earth. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans. The composition of the atmosphere varies, depending on your location, the weather, and many other things. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere.
Pollution can add different gases or dust and soot.
It gradually thins out as you go higher and higher up. There is no sharp break between the atmosphere and space. Light is a kind of energy that radiates, or travels, in waves. Many different kinds of energy travel in waves. For example, sound is a wave of vibrating air.