Q: My friend here in Kenya uses a piece of cloth soaked in a washing detergent to wipe the inside of his vehicle鈥檚 windscreen whenever it rains. This prevents the windscreen from becoming misty, even if the windows remain closed. Why does this work?
A: Mistiness is caused by tiny droplets of water which scatter the light and make the glass opaque. Wiping the glass with a cloth which is soaked in a detergent leaves a residue which lowers the surface tension of the water droplets enough to allow them to spread as a thin film, which is as transparent as the glass.
A: The effect can be seen using a bar of soap to write on a dry bathroom mirror. Once the message is written on the mirror, it can be wiped dry to remove the excess soap and the writing then disappears.
Advertisement
However, when the steam from a hot bath or shower condenses on the mirror, the areas of writing do not mist up, but the background does and the message becomes readable again.
My personal preference is for a romantic message for the person in the bath, but be warned 鈥� the effect will be visible for some time whenever the bathroom is steamy.