How Do I Stop my Car Window Fogging Up?

Winter is upon us.  That means rain.

How do you stop the windscreen in your car fogging up?

It’s pretty easy.  Get a dry cloth, squirt a generous amount of dishwashing detergent on it, and rub it all over the inside of your windscreen.  Initially, of course, it will smear everywhere and make a mess, but just keep rubbing until it becomes clear again (which is when all the water in the detergent dries out).

You will be amazed at how well it works.

Here’s how it works: mist (or for that matter fog) is tiny little beads of water sitting on the glass on your windscreen.  If you could shrink down to microscopic size you would see that they are not actually wetting the windscreen.  They are sitting on top like little individual beads, rather like tiny little golf balls.

This is because water has very high surface tension, as result of unbalanced surface charges, as we have seen.  These will act like lenses and when light comes through them, it gets scattered everywhere.  This is why you cannot see through them very well.

A thin film of detergent, however, helps balance the charges on the surface of the water, and helps it wet onto the windscreen.  That is, all the little beads collapse and spread out over the surface.  So they are no longer acting like lenses, and the light is able to pass through

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