Why do Glasses Fog Up?
When we are walking in cool air and suddenly move into relatively hotter room then the glasses will get fogged up. The water vapor condenses at cool temperature in the atmosphere. Eye glasses get cooled in the outside atmosphere. When it is brought inside the house the moisture present in the room air touch the glasses and gets condensed. This appears as fog. This will also appear on the glass windows of the houses or car glass and so on.
When the warm air comes in contact with the cold glasses it transfers some heat energy to the glass and results in formation of a thin foggy layer it. When the glasses also get heated up then the thin layer on the glasses will evaporate clearing the vision through the glass. The fog disappears due to the warming up of glasses.
The effect of the humidity in the room on the very cool eye glasses brought into the room can be studied by an experiment. If the air inside the room is warm and generates more water vapor, the vapor pressure of the inside air becomes higher. As long as the vapor pressure of the air in the room is higher than the vapor pressure on the glasses, the water vapor in the room air condenses on the glasses and creates fog. By an experiment if we can remove the humidity on the room air and make it dry then the water vapor is not formed and the vapor pressure also do not exceed the vapor pressure on the cool glasses. Then the formation of fog does not occur.
The condensation of the surrounding air on the glasses will occur only when the glasses are cooler below the dew point. The phenomenon of fog formation on eye glasses is similar to that of formation of fog on the bathroom mirror when the air inside the bathroom is warm.