Why does watching magnesium burn leave a dark "spot" in your vision?

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Offline Seany

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Ok. Hopefully everyone knows that when you heat magnesium, you get magnesium oxide. This reaction is huuuuuuuuuuuuuge so you need to wear goggles and also those coloured filter things.

At school, we have always been scared it might damage our eyes, so we all use those filters. What would happen if we didn't?
« Last Edit: 19/05/2007 11:47:46 by chris »
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Offline daveshorts

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Magnesium will burn very brightly and emit quite a lot of light in the Ultra Violet part of the spectrum. I would have thought the danger is that you would get welder's eye. This is a problem welders get if they don't use filters. The Ultra Violet will literally give you sun burn to the retina - a very unpleasent experience and if it is bad it may cause scarring.

Saying this when I did this experiment 17 years ago we didn't use filters...

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Offline Seany

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Don't you see a little black spot everywhere you look? After time, it disappears.. But it is very annoying.. But sometimes, can the black spot stay there forever?
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Offline DoctorBeaver

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There was an incident where I live a week or so ago involving magnesium. 2 toms of it caught fire at the Alcoa aluminium plant. It happened at around 10:15pm and the glow could be seen from miles away.

It seems a furnace door jammed open and the flames said "Oooh look magnesium. Let's have some fun!".
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Offline daveshorts

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The black spots you can see are probably because the light sensitive cells in the back of your retina, work my making a pigment - I think called visual purple. When this is hit by some light it changes chemically and in the process sends a signal down the nerve. This means that if you see really bright light the pigment gets used up so for a while afterwards there is less pigment in the cell, so you get a less strong signal for the same brightness of light.
 This is also why it takes a couple of minutes for your eyes to adjust to low light levels, because there is no light hitting them the level of visual purple will build up until your eyes get much more sensitive