Why does shaving foam stop the mirror from steaming up?

25 July 2010

Question

Hello Guys, 

Very good job! I whole heartedly approve of making science approachable for non-scientsists. You guys manage to do this very well.

When I take a shower the mirror in the bathroom mists up. This is clearly a process of condensation of warm moisture from the shower, condensing on the cool surface of the glass of the mirror. If one wipes the mirror, with say a towel, then the condensation returns relatively rapidly. However, I have noticed that if I rub a small amount of shaving foam onto the mirror surface, then the condensation is removed and no further condensation is formed on the mirror. I would like to know what is happening to cause this effect? Is it something along the lines of a wet t-shirt becoming translucent when wet? Why does the foam get rid of the moisture and how does it prevent further condensation from occurring?  Kind regards

Brett

Answer

Dave - In some senses, it doesn't actually stop it steaming up. Steam is lots of little droplets of water. When the light hits it, the light gets bent and so you get a very distorted image which, when you move away from it, just looks like a kind of mist. What the shaving foam does: there are lots of detergents in it and those detergents reduce the surface tension of the film of water so it doesn't form lots of little droplets. It just forms a big flat sheet that you can see through much better, so you can see through it even though the water is still condensing.

Ben - So there's literally the same amount of water there; it's just a change in the structure of the water. So why do the droplets make it so hard to see things?

Dave - Because water has quite a high refractive index - when light hits it, it bends. If it hits this curved surface of a droplet, each one basically acts as a little tiny lens, light is bounced off in all sorts of different directions and makes it looks essentially white. This breaks up the image and makes it look misty.

Ben - Fantastic! Well very interesting. Not perhaps what you're supposed to use shaving foam for but if it works, it works.

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