David finds out firsthand what happens when film is developed.
Segment Length - 8:03
- How do atoms in photographic film change when exposed to light?
- What happens when the film is developed? Is color film different from black-and-white?
You may have heard that photography is a "snap." That's because every picture begins
with the snap of a shutter. When the shutter opens, light is captured and recorded on
photographic film. It doesn't matter if the film you're shooting is black-and-white or
color--the same chemical processes are at work.
Black-and-white film is made by suspending billions of tiny silver halide crystals in
gelatin and coating a clear sheet of plastic with the gelatin. Each tabular-shaped
crystal contains silver and bromideions held together by
strong electrostatic forces.
Photons of light with the right amount of energy can change
these crystals, producing silver ions, bromine atoms, and free electrons.
An electron wanders through the crystal until it is captured by a silver ion. In the
process, silver ions are changed into black metallic silver.
The number of silver atoms formed depends on the amount of light each crystal receives.
In an exposed crystal, as few as four silver atoms may form. These atoms usually clump
together in tiny dark specks too small to be seen even with the most powerful
microscopes. Although the silver specks remain invisible to the naked eye, they form
a pattern that depicts the photographed subject. This is called a latent image.
In a darkroom, you immerse the film in developer
to make the latent image visible. The developer acts by pushing electrons into the film. The silver specks channel
electrons into the exposed crystals, where more silver ions are changed to black
metallic silver. Unexposed crystals react slowly or not at all with the developer.
In this way, film exposed to light becomes darker than unexposed areas of the emulsion.
This produces a negative.
Then you put the negative into a stop bath, where silver ions stop changing into
silver atoms. That's because the stop bath contains a weak acid that shuts off the
supply of electrons from the developer. Next, the negative goes into the fixer. The
fixer contains a solution (hypo) that dissolves silver bromide salts from the negative,
leaving behind a dark silver image. After a final rinse in water to remove chemicals,
the negative is ready to dry and print.
Color film is made by stacking three layers of black-and-white emulsion. Each layer is
sensitive to one of the three primary colors of emitted light (red, blue, and green).
When exposed to light, color film forms a latent image just as black-and-white film
does. But during development, the developer reacts with other chemicals to form
colored dyes. The dyes form in the emulsion at the exact location of the exposed
crystals. Later, the exposed and unexposed silver is bleached out, leaving dye
molecules that reproduce the color of the photographed object.
- What causes photographic film to fog?
- Why does some photographic film appear grainy?
- Why can't you develop film in the daylight?
- Why does film have to be agitated in the stop bath?
acids substances that release hydrogen ions in water and
developer a substance that changes silver bromide
crystals into black metallic silver
electrons a subatomic particle that carries a single
electrostatic forces an attraction or
repulsion between electrical charges at rest
emulsion silver bromide crystals floating in gelatin.
gelatin a water-soluble protein made by boiling animal
skin and bones
hypo a water solution of sodium thiosulfate
ions charged particles formed when neutral atoms gain or
photon a particle of light
- Bustillo, I.J. (1989, Feb) How to. Modern Photography, pp. 54-55.
- Cyr, D. (1977) Teaching your children photography. New York: American Photographic
- Keller, E. (1970, Dec) Images in color. Chemistry, pp. 8-11.
- Keller, E. (1970, Oct) Images in silver. Chemistry, pp. 6-12.
- Klavan, G. (1991, Mar) Color from a closet. Popular Photography, pp. 42-43.
- Kolonia, P. (1993, Feb) Films of choice. Popular Photography, pp. 30-37.
- Langford, M. (1981) The darkroom handbook (1st ed.). New York: Random House.
- Neblette, C.B. (1970) Fundamentals of photography. New York: Van Nostrand Reinhold.
- Willson, M. (1988, Mar) Make your own black-and-white prints. Astronomy, pp. 70-75.
Additional sources of information
Eastman Kodak Company
343 State St.
Rochester, NY 14650
- Local photographer or photo shop
- School photography darkroom
- Newspaper photojournalist
- Pinhole Photography
Build and operate a simple camera.
Make a photograph using a shoe box and some aluminum foil. Learn how a camera focuses
and controls light. Develop the latent image into a negative.
- shoe box
- black construction paper
- glue stick
- aluminum foil
- photo-enlarging paper
- 3 glass baking dishes
- common paper developer and fixer solutions
- Cut a 2.5-cm (1") square in the top of a shoe box. Line the inside of the shoe box
and its cover with black construction paper attached with glue stick to help cut down
on stray light reflections. Next, cut a 5-cm (2") square of aluminum foil and press
it smooth on a flat surface. Use a straight pin to prick a small hole at the center
of the aluminum foil.
- Now tape the aluminum foil over the cutout opening in the shoe box. Then cut a
7.5-cm (3") square of black construction paper.
- You must load your camera in a darkened room. With the room darkened, cut a sheet
of photo-enlarging paper to fit the bottom of the shoe box. Secure the paper,
emulsion (or shiny) side up, by taping the four corners to the bottom of the box.
Replace the cover on the box and use two rubber bands to hold it in place. Insert
the 7.5-cm square of black construction
paper under the rubber bands and over the pinhole to act as the shutter. Now you are
ready to take a picture.
- Take your pinhole camera out on a sunny day and make a picture by sliding the
shutter (black paper) to one side of the pinhole. A good exposure time is 10 minutes
but you may need to try other exposure times to get the best results with your camera.
Don't bump or move your camera while the shutter is open. Do not hand-hold the camera--
instead, support it on an object such as a rock or chair. After the exposure, close the
shutter and remove the exposed paper in a darkened room. To reveal the latent image,
the paper must be developed.
- In a room that is either totally dark or illuminated by red light, slide the paper
into a tray of paper developer and agitate the solution for two minutes. Remove the
paper and allow it to drain. Next, slide the paper into a tray of fixer to make the
image permanent. Agitate the fixer solution for four minutes, then remove the paper.
Finally, wash your negative in a tray under running water for five minutes.
Blot (do not rub) the negative dry with a paper towel and leave it to dry
- How could you modify the pinhole camera? Can you think of other camera designs?
- How does making a pinhole camera help us understand the workings of commercial
- How would changing the size of the pinhole affect the image? Does the distance
between the photographic paper and the pinhole make any difference? Why?
- What did the enlarging paper do in the developer solution? What happened when the
paper went into the fixer? What kinds of tools or resources would make developing
Why are different film speeds available? The size of the silver-nitrate grains on the
film determines the speed-- the larger the grains, the faster the film. Research what
the speeds on a film box represent. Which speeds are better for which light condition
s? Then read What's That You See? by B. M. Bolotovsky in Quantum magazine
(Mar/Apr 1993, pp. 4-8). This article explains why shutter speed and film speed
are part of the reason we can't always believe our eyes.
Invite a photographer to come and talk to your class about photography. Ask the
photographer to bring a camera and accessories to demonstrate the equipment and
methods needed for good picture-taking.
Who made the first photograph? Construct a time line to show inventors of photography
from the camera obscura to the present.
Discuss the settings on a 35mm, adjustable camera. How does the camera operate?
Take pictures using different camera settings (shutter speed, aperture) to learn how
to produce the best exposures.
Newton's Apple is a production of KTCA Twin Cities Public Television. Made possible
by a grant from 3M. Educational materials developed with the National Science