Teacher Notes

Camera Obscura

Historical Inventions Laboratory Kit

Materials Included In Kit

Ascorbic acid, 100 g
Citric acid, 100 g
Sodium carbonate, 175 g
Sodium sulfite, 100 g
Black paper, 100 lb, 12" x 18"
Brass strips, 6", 15
Electrical tape, black, 2 rolls
Forceps, 15
Photographic paper, 25 sheets
Push pins, 15
Trays, plastic, clear, 30
Weighing dishes, 75

Additional Materials Required

(for each lab group)
Lemon juice or citric acid
Water, distilled
Beakers, 250-mL, 2
Glue gun

Prelab Preparation

It may be beneficial to weigh out 5 g of ascorbic acid, 5 g of sodium sulfite and 10 g of sodium carbonate beforehand for each lab group.

Safety Precautions

Sodium carbonate causes mild skin irritation, serious eye irritation and may be harmful if swallowed. Sodium sulfate causes eye irritation and may be harmful if swallowed or inhaled. Wear chemical splash goggles and chemical resistant gloves and apron when working with the developer and stopper solutions. Wash hands thoroughly with soap and water before leaving the laboratory.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Leftover developing and stopper solutions may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

  • Best results will occur on sunny days. On a bright day, the camera most likely only needs to be exposed a maximum of 30 seconds.
  • If the school has a photography class equipped with a dark room, it is suggested to inquire about the use of the dark room for this activity.
  • If the classroom has no windows and can be made very dark by covering light sources, it will be suitable as a dark room.
  • If no red light source is available, smartphone apps that provide red-only light can be used. This has been tested successfully in our lab.
  • Leave the photopaper in the box and black plastic it comes in until students are ready to enter the darkroom. In the darkroom, give each group one sheet of photopaper and place the extra sheets back in the original packaging. After students are done cutting out the photopaper, and before turning the light on, collect the extra photopaper from the student groups and put them back in the original packaging.
  • When leaving the darkroom to capture their photos, students must ensure that the camera box remains shut on their way to their preferred photo location so that no light leaks into the box and activates the photopaper.

Teacher Tips

  • The photographic paper is composed of silver halide crystals suspended in gelatin that have been coated onto the paper. The incident photons that strike the paper will promote electrons in the silver halide crystal to the conduction band. The free electrons can move through the crystal lattice and eventually reduce an interstitial silver ion to atomic silver. This leads to the formation of small silver clusters that form a latent image. This can now be put in a developer solution, that will further reduce the silver halides that have been exposed to light.
  • Ascorbic acid is a reductant; meaning it will convert the silver ions in the silver halide crystals in the photographic paper to silver, which gives the appearance of a negative image.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Planning and carrying out investigations
Obtaining, evaluation, and communicating information

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Cause and effect

Performance Expectations

MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Answers to Prelab Questions

  1. What is the ray model of light?

    The ray model of light is an idealized model that states light travels from its source in straight lines or rays.

  2. Read the Procedure. How long must you expose the photographic paper be exposed?

    The paper should be exposed for no longer than 45 seconds.

  3. What sort of care should be taken when handling the photographic paper?

    Great care should be taken to not unnecessarily expose the paper and render it useless. It should only be handled in a dark room.

Answers to Questions

  1. In your own words, explain how a pinhole can be used to form an image.

    The pinhole acts like a small point for individual light rays to cross through. The light rays that come through illuminate the back wall of the chamber. If the pinhole is small enough, the light rays will come from separate points of a light source and will fall, without overlapping each other, upon the surface opposite the pinhole, creating a real inverted image.

  2. What would happen to the image if you recaptured your photo, but the camera was flipped upside down?

    Flipping the camera upside down would not affect the outcome.

  3. What is the focal length of a pinhole camera?

    The focal length of a pinhole camera is infinite.

  4. How would your image change if you captured an image at noon as opposed to 4:00 p.m.?

    The image taken at noon would be brighter than an image taken at 4:00 p.m. due to the most intense sunlight available at noon. The photographic paper would be exposed for a shorter period of time.

  5. (Advanced) For the best image to be produced, the apertures should be 1/100 or less of the distance between it and the projected image.

    d is pinhole diameter
    f is focal length (distance from pinhole to image plane)
    λ is wavelength

    If your friend wants to make his own pinhole camera but has a box of 20 cm x 20 cm x 20 cm, what should be the size of the pinhole to produce the best image?
    d = 2 *sqrt((0.2 m) x (550 x 10 –9 m)) = 6.63 x 10 –4 m.
    To produce the best image, the diameter of the pinhole should be 0.66 millimeters.

Teacher Handouts


Student Pages

Camera Obscura


Develop your own photographs with a self-made pinhole camera! Create a developer solution and use photographic paper to capture amazingly detailed images with simply a box with a pinhole.


  • Cameras
  • Image capture
  • Lenses
  • Photography


Italian scholars such as Giambattista della Porta (1538–1615) and Leonardo da Vinci (1452–1519) were among the first to form images in dark rooms. The dark room used for this was essentially a large closed box with a hole on the side that faced the light source. The light source can be an object that emits light, such as the Sun, or one that absorbs some incident light and reflects the rest so our eyes can see it, such as a tree. If the hole into the room is too big, light will simply shine through to the surface opposite the hole in the room with no image to be seen. However, if the hole is made much smaller, an inverted image of the tree will be seen on the back wall of the room (see Figure 1).

The reason for the formation of the image can be explained by understanding the ray model of light. This is an idealized model used in optics that states that light travels in straight lines or rays. When light rays from the Sun (white light) strike the leaves on a tree, some light is absorbed (like the blue and red light), and the green light rays are reflected in all directions. Some of these reflected rays of light strike your eyes and allow you to see the green leaf. The reason we see green is because that is the only color wavelength that was not absorbed. Now imagine a tree in front of a large closed box with a hole as in Figure 1. A light ray reflected from the top of the tree goes through the hole and strikes the bottom of the back wall in the box. A light ray reflected from the bottom would go through the hole and strike the top of the back wall. A ray starting from the right side of the tree would cross over to the left once it passed through the hole, and vice versa. Thus a real and inverted image would be formed. However, if the hole is too large, it would allow many rays from every portion of the light source enter resulting in a large number of inverted images form at the same time, overlapping each other and thus blurring out into general illumination; no image would form. With a much, much smaller hole, there would be a vast reduction in the number of overlapping images. The image formed with a very small hole would be noticeably sharp.

The type of room originally used to produce these images is called camera obscura, Latin for “dark room.” Over time other devices for producing images within darkened interiors were created and the word “camera” was attributed to all such devices. What is now known as a pinhole camera was the original camera obscura.

In essence, the pinhole works as a lens that is not limited by a need to focus an image; therefore, the focal length of the pinhole is infinite. The pitfall of the pinhole is that in order to increase the sharpness of the image one must keep the hole as small as possible. This means that the total amount of light passing through the hole is small, and the image is dim. To increase the size of the opening to allow more light through, and also avoid the superimpositions (overlapping images) that would prevent an image from forming, a converging lens must be inserted in the opening. A converging, or convex, lens focuses incoming light from a distant object through the back focal point of the lens to form a real image on the transmission side (exiting-light side) of the lens. The focal point of any lens is the point at which a beam of light parallel to the principle axis of the lens converges. A real image is an image that can be formed on a screen and therefore can be seen by the naked eye. Refer to the ray diagrams in Figure 2.
The lens inserted in place of the pinhole will concentrate the light from a large area into focus, greatly increasing the brightness of the image without loss of sharpness. In 1599, della Porta described such a device and invented the camera as we now know it.

Experiment Overview

The purpose of this activity is to learn about the science of photography by building a pinhole camera. A photographic developer solution will be made and photographic paper used to take pictures of the surroundings. Photos will be developed in a darkroom.


Ascorbic acid, 5 g
Citric acid, 5 g
Sodium carbonate, 10 g
Sodium sulfite, 5g
Water, distilled, 200 mL
Beaker, 250 mL
Black paper, 100-lb, 12" x 18"
Brass strip, 6"
Electrical tape, black
Glue gun
Paper towel
Photographic paper
Trays, plastic, clear, 2
Stirring rod
Weighing dishes, 3

Prelab Questions

  1. What is the ray model of light?
  2. Read the Procedure. How long must the photographic paper be exposed?
  3. What care should be taken when handling the photographic paper?

Safety Precautions

Sodium carbonate causes mild skin irritation, serious eye irritation and may be harmful if swallowed. Sodium sulfate causes eye irritation and may be harmful if swallowed or inhaled. Wear chemical splash goggles and chemical resistant gloves and apron when working with the developer and stopper solutions. Wash hands thoroughly with soap and water before leaving the laboratory.


Part A. Developer Solution

  1. Mix 5 g of ascorbic acid with 5 g of sodium sulfite with 200 ml of distilled water.
  2. Add 10 g of sodium carbonate to the solution and mix thoroughly.
  3. Pour the solution into one of the clear plastic trays.
  4. For the stopper solution 5 g of citric acid with 200 ml of water and pour into the remaining empty clear plastic tray.
Part B. Building the Pinhole Camera
  1. Use the template provided with this activity.
  2. Glue the template to the black paper.
  3. Carefully cut out the template.
  4. Using the push pin; score along the dotted lines of the template.
  5. Fold along the score lines.
  6. Cut a 2 cm x 2 cm center space for the pinhole shutter.
  7. Cut out a 1" length of brass from the brass strip.
  8. Obtain the push pin and rotate its needle in the center of the brass strip until the pin just goes through.
  9. Attach the brass pinhole plate to the inside of the camera box using black electrical tape. Note: The pinhole plate should be placed so that the pinhole is at the center of the hole cut in step 6. Do not cover the pinhole with tape.
  10. Glue the edges of the camera template and assemble the camera box. Note: The box will have five closed faces and one open face.
  11. Cover the edges of the camera box with black tape to minimize light leakage.
  12. Cut out a 2" x 2" square of black paper. Attach one edge of the square to the front of the camera with black tape so that it covers the pinhole. This will be the camera’s shutter flap (see Figure 3).
Part C. Capturing an Image
  1. Cut out a 2" x 3" piece of photographic paper. Note: Do not expose the photographic paper to light.
  2. In a completely dark room, attach a small tape roll on the back of the photographic paper and placing the paper glossy side up in the center of the back of the camera.
  3. Completely seal the camera.
  4. Ensure the shutter is closed over the camera before leaving the dark room.
  5. Take your camera outside and determine what you will take a picture of.
  6. When taking the picture, the camera and the subject of the picture should remain very still.
  7. Carefully but quickly remove the shutter from the front of the camera and hold it very still for an exposure that lasts about 45 seconds but no longer than a minute. Quickly cover the pinhole with the shutter.
  8. Return to the dark room.
Part D. Developing the Photograph
This must be done in complete darkness. Use a red light source to help you see in the dark room. The photographic paper is not sensitive to these types of light.
  1. Set out in front of you the two plastic trays that contain the developer solution and stopper solution and plastic forceps.
  2. Take the photographic paper out of the camera, hold it by the edge with the forceps and dip it in the developer bath.
  3. After about 30 seconds, check the paper for the appearance of dark spots. If it doesn’t look ready, dip the paper in the solution for another 30 seconds.
  4. Using the forceps, take the photo out of the developer solution and place it in the stopper solution for 30 seconds.
  5. Take the photo out of the stopper solution with forceps and turn on the light. You should now have a negative image of your photo.
  6. Lightly pat dry with a paper towel and set out to dry.

Student Worksheet PDF


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