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Science 7–10 · Year 9

Pinhole camera: a model of how the eye forms an inverted image

Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area; serves the content point on how the eye responds to light; the straight-line travel of light is introduced in Year 5, AC9S5U03, which also names reflection and refraction, and the ray model the learner draws in step 6 is one of the light models AC9S9U04 asks Year 9 learners to use, so AC9S9U04 is cited here as it is on the reflection, refraction and lens entries)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Rays from each point of an object travel in straight lines through a small opening and cross there, so the image on the screen is inverted and its size depends on the distances. The eye forms its image on the retina in the same way, with the pupil as the opening and the lens gathering enough light to keep the image bright and sharp.

What you need

  • cardboard tube or box 10 cm deep with a tracing-paper screen at one end and foil over the other, 1
  • pin, 1
  • bright object: a lamp behind a 20 cm cut-out letter F, 1
  • metre rule, 1
  • darkened room

How to do it

  1. Make a single clean pinhole in the centre of the foil. Point the camera at the illuminated letter from 1.00 m and look at the tracing-paper screen from behind.
  2. Record whether the image is upright or inverted and left-right reversed, and measure its height.
  3. Move the camera to 0.50 m and 2.00 m from the object and measure the image height each time.
  4. Plot image height against 1 / object distance.
  5. Make a second pinhole 1 cm from the first and record what happens; then enlarge one hole to 3 mm and describe the image.
  6. Draw a ray diagram with two rays from the top and bottom of the object through the pinhole to the screen, to scale, and predict the image height for 1.50 m.
  7. Compare the camera with a labelled diagram of the eye: match the pinhole to the pupil and the screen to the retina, and explain why the image formed on the retina is inverted.

What you should see

The image is inverted and reversed. For a 20 cm letter at 1.00 m and a 10 cm deep camera the image is 2.0 cm high; at 0.50 m it is 4.0 cm and at 2.00 m 1.0 cm, so image height is proportional to 1 / distance. Two pinholes give two overlapping images; a 3 mm hole gives a brighter but blurred image. The scale drawing predicts 1.33 cm at 1.50 m, and the camera's image is close to it. The eye's image on the retina is inverted in the same way, and the brain interprets it so that the world is seen upright.

What changes

What you change
object distance (m) or pinhole size
What you measure
image height (cm) and sharpness
What you keep the same
  • same camera depth
  • same object
  • camera pointed straight at the object

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • The pinhole makes the image, so a bigger hole makes a better image.
  • Light from the object spreads out and lands upright on the screen.
  • We see objects because light comes out of our eyes.

Safety card

Low riskLearners carry it out

Hazards

  • pin
  • dark room

Controls

  • lamp only, no candle, so there is no naked flame
  • pin stored in a cork when not in use
  • torches for moving about

Note

Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
  3. spark.iop.org/collections/pinhole-camera-and-lens-camera
  4. instructional-resources.physics.uiowa.edu/6a6121-pinhole-camera-pinhole-images

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