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Magnifying Glass & UV

  1. Jan 27, 2010 #1
    I was focusing sunlight on my Magnifying Glass & was amazed how powerful the beam was. Even on a dark object it was far too bright to look at. I wanted to try & cook & marshmallow with it. But would that be dangerous because of uv rays? Is it just visible light that focuses through the glass or uv too?

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  3. Jan 27, 2010 #2

    Claude Bile

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    The magnifying glass will cause UV light to focus just as it would visible light (though the focal point will probably be different due to chromatic dispersion in the lens). The main difference between focusing visible and UV light is that glass is typically much more transparent in the visible.

  4. Jan 27, 2010 #3


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    Just to expand on this, the UV light that gets through a standard glass magnifying glass is UVA, the least energetic and damaging kind. Shorter wavelengths (UVB and higher) will be absorbed by the glass (this is why it is hard to get sunburned indoors). Feynman famously claimed watched the first ever nuclear test with his naked eyes from (far away) inside the cab of a truck, because he trusted the glass to absorb the short wavelength radiation that would have harmed his eyes.

    Also, whatever you are "cooking" will be absorbing a lot of the focused UV light (which will help with the cooking). Still it is a good thought, and you should wear sunglasses while you do it to protect your eyes, just in case there is some scattered/reflected UV.
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