White fogginess on glasses/cups!

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Offline Seany

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White fogginess on glasses/cups!
« on: 15/06/2007 15:33:36 »
I have seen lots of these foggy white things on my cups!! They make the cups look filthy..

However, on the book "Naked Science" it explains that it is the impurity of the glass.. And that the holes/cracks widen in the dishwasher, and the light bounces about, rather than going straight through the transparent glass.

So why does this make it white?
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Offline chris

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White fogginess on glasses/cups!
« Reply #1 on: 16/06/2007 12:17:30 »
Hi Seany

I'm impressed - you've read the book!

The explanation is the same as for the questions "why is snow a white colour" when water (which it's made from) is clear, why is sand opaque when glass (which is made from sand) is transparent, and why is light-coloured hair darker when it's wet?

So let's consider the snow situation, and then apply that to the foggy wine glass.

Water, like glass, is translucent to light, which means that when light enters water it changes direction slightly. This is why a straw in a drink looks bent when you look at it from above.

But snow is made of billions of tiny ice crystals, which are each individual blobs of water. So light entering snow gets bent in one direction, then another and then another until it ends up coming back out the way it went in i.e. it gets reflected back at you. It's white because all wavelengths of light are equally reflected so they combine to make you see white light.

Now consider the foggy wine glass. The imperfections in the crystal structure of the glass make it start to behave like the snowflakes we have just discussed above. Most of the light entering the glass still goes straight through, but some gets bounced about and reflected back at you, just like with the snow. The result is a white "bloom".


It's because the imperfections introduced into the glass over time by the dishwasher cause photons of light to ricochet around inside the glass and bounce back at you.
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