[Leica] The formation of the latent image - it ain't easy!

[Edward Caliguri <caliguri@rcn.com>]

Martin-
    Again this can get very involved  - but from a photographic point of
view, Silver, or Ag (usually chloride in films - or bromide as well in
papers) will, as I had said, along with other metallic salts of palladium or
platinum, is changed in electronic structure when light strikes it. You can
look at it from ewo frames of reference; light as a wave (like a radio wave)
or a particle - or 'quanta' - which travels very fast, has almost no mass,
but carries a lot of energy with it. Either way this energy gets imparted
(it cannot be made or destroyed, only changed) to the Ag atom. Now, the Ag
atom in a chloride salt, AgCl, is in a +1 state - meaning it has 'given an
electron to the chloride to make this compound. The nature of the AgCl
molecule, it's MOLECULAR electronic orbital structure, enable this energy to
be taken in, and 'held' by the Ag atom in the compound, which, because it is
in the +1 state, has an extra un-filled orbital, by giving the outermost
electron in Ag enough 'umph' to occupy this higher orbital state. It likes
it there, and stays there - forming the 'latent image' (there is a good
quantum mechanical reason for this - but take it on faith!). If a developer,
or a reducing agent (a chemical that wants to get rid of an electron to
become more 'stable') comes in contact with this activated Ag in an alkaline
solution, it will add an electron to the silver, pushing the energized
electron 'back in its place', and will take over it's former place in the
elevated orbital. This allows Ag+1 to become Ag metal, or silver with NO
charge. The Chloride it was formerly paired with goes off hand in hand
happily with the reducing agent, but the silver metal is held in the
emulsion. The Unexposed Ag stays in the salt form, later to be removed by a
solution of thiosulfate, or fixer, by what is termed s complex ion formation
(a whole other topic) - but the silver metal stays - the negative!! Color
uses the same principle, however adds layers of dyes and complexing agents.
I can give you the names of some textbooks off list, if you like!
Ed

> From: owner-leica-users@mejac.palo-alto.ca.us (Leica Users digest)
> Reply-To: leica-users@mejac.palo-alto.ca.us
> Date: Sun, 22 Jun 2003 12:28:40 -0700 (PDT)
> To: leica-users-digest@mejac.palo-alto.ca.us
> Subject: Leica Users digest V25 #5
> 
> on 06/21/03 5:00 PM, Martin Howard at mvhoward@mac.com wrote:
> 
>> My question has to do with *how* "the energy of the light changes the
>> chemical structure of the silver nitrate or whatever it is".  Brain's
>> links provided a starting point.

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