From: SFPhysics@aol.com
Date: Sat Dec 09 2000 - 01:20:30 PST
From: SFPhysics@aol.com Message-ID: <4e.e795eba.2763535e@aol.com> Date: Sat, 9 Dec 2000 04:20:30 EST Subject: Re: Black Light (UV)
> My 7th grade science class is studying light. A lot of the kids keep
> asking about black light. My understanding and limited research is that
> it is UV light - beyond the visible spectrum. Then how come we can see
> it? Why do flourescent colors and white glow? What is black light? I
> would appreciate some info., thanks, Andrea
>>
Hello Andrea:
You cannot see UV light. The "blacklight" bulbs we use in class for making
flourescent materials glow has not filtered out the extreme end of our vision
in the violet region so we see part of the lamp's output. However, the
majority of energy is in longwave UV which we cannot see. A professional
laboratory grade UV source would not be seen at all.
Flourescent materials glow under UV because they are taking in the energy of
the UV and giving it back in visable light. The process is one of atoms and
molecules taking in a photon at UV light energy levels then kicking out
photons of lower energy that are visable in a very narrow part of the
electromagnetic spectrum where we have our eyesight. DayGlo materials and
white shirts operate on this principle. Both absorb high energy photons that
drive up electrons into higher energy states. When the electrons drop back
down to a rest state the energy is given off as a photon.
I like to explain UV flourescence to lower grade students as when someone
lifts a book up then the book is dropped on the desk to make a sound. The
lifting is done by energy (a photon in a UV flourescent material) and the
sound energy caused by the book drop is another energy (a photon of different
wavelength). You may not hear the energy in the lifting book but you do hear
the energy in the dropped book's colision with the desktop. Same idea but
with eyes not ears!
Hope that helps,
Al Sefl
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