After a shower or using hot water from the sink, the mirrors in a bathroom will appear foggy or misty. This is caused by condensation of water vapor in the air from gas form into small water drops.
These drops are noticeable on glass materials because this type of materials distort the light waves, making the glass look misty. The foggy appearance is mostly more visible depending on the degree of hotness of the water used. The hotter the water, the more further its molecules are placed apart.
When the heat from the shower water warms up the air in the bathroom and since the cooler air cannot hold too much water, tiny drops of water will be suspended in mind air until they become saturated and fall to the ground. In reality, everything in the bathroom becomes foggy including the walls and the ceiling but this is only visible on glass surfaces due to light bending.
As the tiny drops of water contact cooler surfaces, they become cool and condenses into water on the glass surface. Glass does not heat up as quickly as air so if the glass surface is cooler than the water accumulated on it, streams of water start running down the glass surface. Light passing through the surface of the water bends, creating a foggy appearance.
However, using a fan with an outlet when taking a shower will help to avoid this vapor accumulation. The fan will keep the bathroom mirrors clear and also stops mildew development. The other option is taking a shower with a door open.
To clear the already built up vapor, open the bathroom door to dissipate the steam. Use a hair drier to blow on the mirror to get the lid off the fog quickly. There are also fogless mirrors in the market that heat up themselves to match the heated air in the bathroom