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Re: lite questions


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From: Dr. William Newbolt <NewboltW@madison.acad.wlu.edu>
To: phys-l@atlantis.uwf.edu
Subject: Re: lite questions
Date: Thursday, December 11, 1997 2:56 AM

Date: Wed, 10 Dec 1997 20:10:41 -0800
Reply-to: phys-l@mailer.uwf.edu
From: fredb@teleport.com
To: phys-l@atlantis.uwf.edu
Subject: lite questions

I have a couple of questions related to a light unit we are
doing.
1) If glass blocks ultraviolet light, why do glasses
need to be uv
protected.
2) If black lights are give off ultraviolet light,
why aren't they
harmful to our eyes? Or if they are harmful, why are they
used so much?

I guess I should answer that glass blocks some ultraviolet
light--knocks down the intensity some. The amount of
attenuation depends on the wavelength of the ultraviolet
light. Also, black light is harmful to unprotected eyes.
Tanning lamps with a lot of ultraviolet used in
environments with very little visible light can burn the
eyes and make them very painful. Don't ask me how I know!
wbn

Another key in the answer is that "glasses" these days are most likely
made out of plastic.

The answer boils down to the fact that not all ultraviolet is the same.
The human eye has a wavelength cutoff in the violet at approximately
400 nm. So something like 390 nm may be ultraviolet by definition, but
is only marginally more harmful than blue light. The smaller the
wavelength,
the more harmful the radiation. For a glass or plastic to be clear for all

visible colors, its cutoff must be somewhere in the ultraviolet region.
The
amount of harmful uv radiation getting through depends on exactly where.


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