How to stop the bathroom mirror from fogging

I have just read Clouds in A Glass of Beer by Craig Bohren – a book about atmospheric physics. It’s a pretty good book, not a total classic that I would recommend to anyone, but good all the same.

It contains some nice experiments which you can do yourself. One particularly useful thing I learnt, is how to stop the bathroom mirror from fogging. Simply smear the mirror with a small amount of detergent, say washing up liquid or liquid soap.

With liquid soap or detergent make a figure on the dry bathroom mirror. Then hop in the tub and take a good hot bath. The moist air of the bathroom is much warmer than the dry mirror. Water will therefore gradually condense onto it. The fogged part of the mirror is covered with small water droplets, whereas the clear part is covered with a more or less continuous film of water. The thin coating of detergent reduces the [surface tension so] that water condenses onto the mirror as a thin film rather than independent droplets. The water droplets scatter light in all directions [causing the fogging]; the water film scatters light mostly in a few directions [acting like a mirror].

Every now and then Bohren produces a really good quote too:

In science there is no authority other than observation and experiment illuminated by reason.

A cloud in many ways is just the inverse of a bubbly glass of beer.

When theory and experiment disagree, it is the theory that must be judged the loser.

Only fools choose to make something more complicated than it need be.

The last point something I feel software engineers could do with taking considerably more notice of.