Optical junk drawer
Build a simple pinhole projection to see why an image forms upside down and how distance changes its size.
GUIDED LESSONStart here
Use this tabletop lesson to observe image formation without a lens. It is a physical experiment you carry out, not an on-screen optical simulator.
By the end: Observe an inverted projection and predict how its height changes when the hole-to-screen distance doubles.
What you need: A light-tight cardboard box, tracing paper for a screen and foil with a tiny pre-made hole. Use a bright ordinary scene; keep the Sun outside the view.
Before you begin: A cardboard box, tracing paper, foil and a tiny pre-made hole. Use an ordinary bright room scene, never the Sun.
How to work through this lesson
- Put the tracing paper at one end of the box and the small foil opening at the other. Block stray light and shade the screen.
- Aim at a bright window frame with the Sun outside the view. Record the orientation and height of the projected frame.
- Increase the hole-to-screen distance while keeping the viewing direction fixed. Compare the image size and brightness, then check the distance calculation below.
Make an image with a pinhole
A pinhole admits narrow bundles of light from each part of a scene. Light from the top of the scene reaches the lower part of the screen, making an inverted image. Unlike a lens, the hole does not focus by refraction.
Hole size is a tradeoff: a large hole admits more light but mixes rays from nearby scene points; an extremely small hole introduces diffraction and becomes dim. Start with a visible image rather than chasing a supposedly perfect hole.
Worked example
Original tabletop experiment: put tracing paper over one end of a light-tight box and the small foil opening at the other. Point it at a bright window frame, keeping the Sun outside the view. Shade the viewing screen. Record whether the frame is inverted. If you double the hole-to-screen distance with the same view, the image becomes approximately twice as large, though dimmer.
Try it yourself
Write your answer before opening the explanation. Use the worked example if you get stuck.
Question 1
A 3 cm-tall projected shape appears with a 10 cm hole-to-screen distance. Predict its height at 20 cm if the distant scene and viewing direction remain fixed.
Show answer and reasoning
Approximately 6 cm by similar triangles: 3 × 20/10. This geometric model does not predict brightness or sharpness, and a nearby subject makes the fixed-scene assumption more important.
Watch for: Light leaks can wash out the image. Seal the box before enlarging the hole. Never use the assembly to view the Sun or a laser.
Summary + next step
Observe an inverted projection and predict how its height changes when the hole-to-screen distance doubles.
Keep the result: Keep a setup sketch, the two distances and measured projection heights.
Sources and further reading
Read these references for the concepts used in the lesson.
Lesson updated 2026-09-14 · Guidance updated 2026-10-22
Coverage and further development
Observe an inverted projection and predict how its height changes when the hole-to-screen distance doubles.
Further development: Documented pinhole builds, prism experiments and contributed optical contraptions.