To understand the property of gases, it is essential to consider them using kinetic theory of gases. This theory assumes that gas atoms/molecules are freely travelling and frequently changing their speed and direction by collisions each other. The purpose of the present article is to try to introduce the basic concepts of kinetic theory of gases to readers. We also show how it explains the fundamentals of vacuum science such as idealgas laws, origin of pressure, surface impinging rate and mean free path.
