Chemistry of Film Photography?

tjc45

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I'm doing a project for Chemistry on photography. I understand that silver halide crystals in the emulsion layer of the film are sensitive to light, but I don't understand what happens after that? What happens to the grains after the photons hit it? Also, what does this formula mean:

Ag[SUP]+[/SUP]Br[SUP]-[/SUP] (crystal) + hv (radiation) --> Ag[SUP]+[/SUP] + Br+ e[SUP]-[/SUP]

THANKS!
 

timor

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Hi. This might help ?
[h=1]"Latent Image Formation
[/h][FONT=arial,helvetica,sans-serif]The light-sensitive part of a photographic emulsion consists of a myriad of tiny (about a micron) crystals of silver halide (mostly bromide). These crystals are suspended in a medium consisting mainly of a very pure form of gelatin (the same stuff that is in Jell-O). The resulting emulsion is thinly spread on a supporting substrate such as a glass plate or plastic film. Actual photographic products have additional constituents and features, but this is the essential form. [/FONT]
[FONT=arial,helvetica,sans-serif] The atoms in a silver halide crystal exist as ions. Each bromine is a negative ion with an extra electron. Each silver is a positive ion with one electron missing. The combination, of course, has no net electrical charge. [/FONT]
[FONT=arial,helvetica,sans-serif] During an exposure to light, a photon event occurs when an incoming photon knocks off the extra electron from one of the bromine ions. This photoelectron is now free and mobile, and it wanders about in its crystal until typically it finds a dislocation or flaw in the crystal lattice, where it becomes trapped. [/FONT]
[FONT=arial,helvetica,sans-serif] A few of the silver ions are not locked in the crystal lattice, and these interstitial ions are also mobile. The negative electric field set up by the trapped photoelectron draws one of these positive silver ions over to it. When the two meet, the ion and the electron combine to form a neutral silver atom. This silver atom is the beginning of a latent image.[/FONT]
[FONT=arial,helvetica,sans-serif] As the exposure continues, the same process is repeated several times until, in at least one place in the crystal, a compact group or aggregate of about four to six neutral silver atoms is eventually created. When this happens, a threshold is crossed. This clump of silver atoms is now large enough that it has become a fully formed latent image. [/FONT]
[FONT=arial,helvetica,sans-serif] A silver halide crystal containing a fully formed latent image has become developable. Developable means that when an exposed plate or film is placed in a developing solution, the latent image aggregate acts as a development center. Starting around the clump of silver atoms, the chemical action of the developer progressively converts the whole crystal into a grain of metallic silver. The developer can be thought of as a chemical amplifier that multiplies the size of the original clump of silver atoms formed by light.[/FONT]"
A citation from Camerabooks.com
How Photographic Emulsions Work: - CameraBooks
 
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tjc45

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Hi. This might help ?
"Latent Image Formation


The light-sensitive part of a photographic emulsion consists of a myriad of tiny (about a micron) crystals of silver halide (mostly bromide). These crystals are suspended in a medium consisting mainly of a very pure form of gelatin (the same stuff that is in Jell-O). The resulting emulsion is thinly spread on a supporting substrate such as a glass plate or plastic film. Actual photographic products have additional constituents and features, but this is the essential form.
The atoms in a silver halide crystal exist as ions. Each bromine is a negative ion with an extra electron. Each silver is a positive ion with one electron missing. The combination, of course, has no net electrical charge.
During an exposure to light, a photon event occurs when an incoming photon knocks off the extra electron from one of the bromine ions. This photoelectron is now free and mobile, and it wanders about in its crystal until typically it finds a dislocation or flaw in the crystal lattice, where it becomes trapped.
A few of the silver ions are not locked in the crystal lattice, and these interstitial ions are also mobile. The negative electric field set up by the trapped photoelectron draws one of these positive silver ions over to it. When the two meet, the ion and the electron combine to form a neutral silver atom. This silver atom is the beginning of a latent image.
As the exposure continues, the same process is repeated several times until, in at least one place in the crystal, a compact group or aggregate of about four to six neutral silver atoms is eventually created. When this happens, a threshold is crossed. This clump of silver atoms is now large enough that it has become a fully formed latent image.
A silver halide crystal containing a fully formed latent image has become developable. Developable means that when an exposed plate or film is placed in a developing solution, the latent image aggregate acts as a development center. Starting around the clump of silver atoms, the chemical action of the developer progressively converts the whole crystal into a grain of metallic silver. The developer can be thought of as a chemical amplifier that multiplies the size of the original clump of silver atoms formed by light."
A citation from Camerabooks.com
How Photographic Emulsions Work: - CameraBooks

Sorry, I don't completely follow. Can someone put this in a simpler form? :confused:
 

amolitor

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Probably not. All that can really be done is to define all the terms, which is a lot of definitions.

Anyways. The point of a silver halide crystal is that it is inherently unstable. Hit it with light, and it breaks down a little into pure silver and pure halide. Just a little, usually (there are cases like "printing out paper" where it breaks down more thoroughly in normal usage).

Developer also breaks down silver halide crystals into pure silver and either pure halide of some other halide compound.

Developer has an easier time breaking down the crystals where light has given it a head start by breaking it down a little already.

So, if you develop for the right amount of time, the developer will mostly break down crystals that got hit with a bunch of light, and will mostly NOT break down the crystals there were left in the darkness.

Now you've got lots of pure silver where there was light, and very little where there wasn't.

Fixer dissolves silver halides, but not silver. Fixer leaves just the silver behind.
 
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tjc45

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Basically, a photon reacts with a crystal and forms a few pure silver atoms, then the developer amplifies the silver to make it visible and the fixer removes the remaining crystals?
 

amolitor

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Yep. That's pretty much it.
 
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tjc45

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So this formula is correct?

Ag[SUP]+[/SUP]Br[SUP]-[/SUP] (crystal) + hv (radiation) --> Ag[SUP]+[/SUP] + Br+ e[SUP]-[/SUP]
 

amolitor

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I think that's implied by the discussion timor posted, yep.

That happens, and then later on tAg+s and the e-s get together to make Ag, which is sort of the point.
 

compur

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None of the above is correct. Images on film are produced by magic.
 

gsgary

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Basically, a photon reacts with a crystal and forms a few pure silver atoms, then the developer amplifies the silver to make it visible and the fixer removes the remaining crystals?

Who gives a ****ing **** as long as it has nice tonality
 

Overread

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Basically, a photon reacts with a crystal and forms a few pure silver atoms, then the developer amplifies the silver to make it visible and the fixer removes the remaining crystals?

Who gives a ****ing **** as long as it has nice tonality

You will when you move on from film and move to wet slide photography or something ;)

Though as an aside lets keep the silly "who cares about the science" comments out shall we. We might not all enjoy chemistry and physics, but lets not discourage those who wish to know more from doing so. :)
 

amolitor

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The anti-intellectual streak in TPF gets awfully damn tiresome at times.

Sometimes, I just want to tell these bozos to grow the **** up and put in a god damned sock in it, because while if you want to be a ignoramus that's ok it's not ok to try to stop other people from not being ignorant fools.

But I don't, because, basically, I am a nice guy.
 

compur

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The anti-intellectual streak in TPF gets awfully damn tiresome at times.

Sometimes, I just want to tell these bozos to grow the **** up and put in a god damned sock in it, because while if you want to be a ignoramus that's ok it's not ok to try to stop other people from not being ignorant fools.

But I don't, because, basically, I am a nice guy.

:lol:
 
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tjc45

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meanwhile I STILL DON'T UNDERSTAND
 

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