The light energy causes electrons to be ejected from a few of the bromide ions, oxidizing them to elemental bromine.
Developed film is a positive film
The negative is dark where Ag+ions have been reduced to metallic silver. Light projected through the negative is captured on photographic paper as a positive image.
Lay some wax paper on the back of an open unloaded camera. Hold the shutter open and
then focus. You have an image. Let the shutter close, however, and the image is
gone. This is the same image that forms on the photographic film inside a loaded
camera. The difference between the film and the wax paper is that the film is able
to retain the image after the shutter has closed. How does it do that? The answer
has to do with oxidation–reduction chemistry.
A simplified explanation of how a black and white photograph is produced is given below:
1. Un‐exposed black and white photographic film is a transparent strip of plastic,
coated with a gel containing micro crystals of silver bromide, AgBr. Light reflected
from the subject being photographed passes through the camera lens and is focused
on these micro crystals. The light causes many of the bromide ions in the micro crystals
to oxidize. The electrons set loose by this oxidation are transferred to the silver
ions, which are thereby reduced to opaque silver atoms. The more light received
by a microcrystal, the greater the number of opaque silver atoms formed. In this
way, the photographic image is encoded, and the film is said to be exposed.
2. The light reflected from the subject does not typically result in the formation
of enough silver atoms to make a visible image. The more silver atoms a microcrystal
contains, however, the more susceptible it is to further oxidation–reduction reactions.
To make a visible image, the photographer puts the film in a light–tight container
to prevent further exposure. Then the film is treated with a reducing agent, such
as hydroquinone, C6H6O2, which reveals the encoded
image by causing the formation of many more opaque silver atoms. Through this step
the image develops.
3. The reduction of silver ions by the hydroquinone developing solution is stopped
by treating the film with a solution of sodium thiosulfate, Na2S2O3 (also called either hypo or fixing solution). The thiosulfate ion,
S2O32–, binds with any un‐reduced silver ions to form a water–soluble salt.
Subsequent washing with water removes everything except the silver atoms adhering to the film,
which are most abundant where the greatest amount of light hit the film when the photograph was taken. The film is now fixed.
4. Because the silver atoms are opaque, the film appears as a negative,
which is dark where the subject was light and light where the subject was dark.
5. Light is projected through the negative onto photographic paper, which is developed using the same reactions that produced the negative.
The resulting developed image is a negative of the negative – in other words, a positive print.
Color photographic film is coated with a variety of chemicals that respond to light of different frequencies (colors).
There are more oxidation–reduction reactions involved in the developing a color photograph, but the basic principle is the same,
the selective reduction of only those chemicals exposed to light. Digital photography, by contrast,
is an outgrowth of photo–voltaic cells, which are made of metalloids, such as silicon, that lose electrons upon exposure to light.