From: SFPhysics@aol.com
Date: Mon Feb 14 2000 - 02:48:49 PST
From: SFPhysics@aol.com Message-ID: <8.13e0b68.25d93791@aol.com> Date: Mon, 14 Feb 2000 05:48:49 EST Subject: Photoelectron Physics
> I have a few questions on how light reduces substances. When light turns
> silver halides black or purple I understand that the silver is reduced, but
> what is oxidized, the halide? Any other details of the chemistry of
> photography would be appreciated.
> Also when a copy machine plate is charged up by light, where are the
> electrons coming from?
> Thanks
> David Lauter
>>
Hello David:
The silver halides (chloride, bromide, and iodide) when hit with an energetic
photon of the correct energy will cause the chemical bond between atoms to
break. It is the developer that reduces the silver ions so they may be
captured by silver sulfide which is also in the film emusion. Going from
basic to acidic, the fixer bath uses acid to wash away the remaining
unexposed silver halide crystals.
AgCl + photon ---> Ag(+1) + Cl(-1) then:
Ag(+1) + developer(-1) ----> Ag(0) reduded metalic silver/dark area of light
exposure
[Remember the definition of reducing agent, anything that will cause the loss
of oxygen, a gain of hydrogen, or a gain of electrons.]
The photosensitive drum in the dry (xerography) copier is coated with a
selenium layer. When a photon of the correct value hits the electron cloud
of a selenium atom, a quantum jump occurs to free an electron. The surface
is not conductive so the free electron ends up as a static charge where the
photon hit. As the uncharged paper passes the drum the paper picks up the
charge and on to the next stage where electrostatic ink is drawn to the
charged areas of paper.
Al Sefl
Guaranteed to get everything 50% correct.
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