show #1210

David finds out firsthand what happens when film is developed. Segment Length - 8:03


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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.


acids substances that release hydrogen ions in water and taste sour

developer a substance that changes silver bromide crystals into black metallic silver

electrons a subatomic particle that carries a single negative charge

electrostatic forces an attraction or repulsion between electrical charges at rest

emulsion silver bromide crystals floating in gelatin. Photographic film.

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 lose electrons

photon a particle of light


Additional sources of information

Eastman Kodak Company
343 State St.
Rochester, NY  14650
(800) 242-2424

Community resources


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.


  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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 overnight.


  1. How could you modify the pinhole camera? Can you think of other camera designs?

  2. How does making a pinhole camera help us understand the workings of commercial cameras?

  3. 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?

  4. 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 easier?


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 Teachers Association.