The fog on glasses consists of small droplets of liquid water. This happens because water vapour from the air condenses on the cold glass surface. The droplets are thick enough to distort our vision. There are two ways the problem can be approached. Heat up the surface, as is done with bathroom mirrors in some expensive hotels, or prevent the condensed water from forming droplets. This can be achieved by lowering the water's surface tension; the force with which water molecules are attracted to each other. The reason water forms beads in the first place is because the attraction of water molecules for each other is greater than for the surface. The idea then is to introduce some substance that gets in between the water molecules and prevents them from sticking to each other. Instead of forming droplets, the water will spread into a thin film which does not obstruct vision. Several substances can achieve this. Isopropanol, or rubbing alcohol, is used most commonly but a variety of detergents can also be used. Rubbing a bit of soap into the glass also prevents fogging.
