Light: Shadows and Reflections · Lesson 4 of 6
Images in a Pinhole Camera
“Make an image on a screen and see why it appears upside down.”
• Describe the source, pinhole, and screen in a simple pinhole setup. • Trace light from the top and bottom of an object to explain image inversion. • Build and adjust a sliding-box pinhole camera. • Compare a screen image with a plane-mirror image. • Observe the image’s orientation and colours without confusing it with a shadow.
An Image Made Through a Tiny Hole
A mirror is one way to see an image; a tiny opening provides another. Place a candle in front of cardboard with a small hole and put a screen beyond the cardboard. In a dim room, light from the candle passes through the hole and forms a recognisable image on the screen. Unlike a simple dark shadow, this screen shows information from different parts of the luminous object.
A device in which light from an object passes through a very small opening and forms an image on a screen inside or behind the device.
The screen image of the candle flame appears upside down. Watch what happens if the cardboard opening is made too large: many different paths from each point can overlap, making the image less clear. A small hole selects a narrow set of paths. A bright object and a dim viewing area make the image easier to observe.
Why the Image Is Inverted
Trace light from the upper part of the flame through the pinhole. It continues straight to a lower place on the screen. Light from the lower part crosses at the hole and reaches a higher place on the screen. The paths cross because only a small opening is available, so top and bottom exchange positions in the screen image. This is an explanation of the observation, not a claim that the flame itself turns over.
Problem
A tree is seen through a pinhole camera. Where will its top appear on the screen?
- 1.Follow a straight path from the top of the tree toward the tiny hole.
- 2.Continue that path beyond the hole to the opposite, lower part of the screen.
- 3.The tree’s top appears toward the bottom of the image; the image is upside down.
Problem
A candle stands upright in front of a card with one pinhole. A student sketches an upright flame on the screen. What should be corrected?
- 1.Identify the two ends of the flame, top and bottom.
- 2.Trace each straight route through the pinhole: the routes cross at the opening.
- 3.Swap their screen positions. The flame image should be inverted, while the candle remains upright.
The chapter’s candle demonstration requires adult supervision. For outdoor use, point the camera at a well-lit tree or building, never look through it directly at the Sun, and do not use the camera to stare at a very bright source.
Construct a Sliding-Box Camera
Choose two cardboard boxes so that one can slide snugly inside the other. Open one end of each. Make a small pinhole in the far face of the larger box. Cut a square opening about 5–6 cm across in the far face of the smaller box and cover it with tracing paper. This translucent paper is the screen. Slide the smaller box into the larger one so the screen is inside and the open end remains available for viewing.
Face the pinhole toward a distant tree or building in bright daylight. Cover your head and the open viewing end with a dark cloth to reduce unwanted light. Slide the inner box forward and backward until the image can be seen clearly on the tracing-paper screen. Record whether you see the object’s colours and whether the picture is upright or inverted. Sliding changes the hole-to-screen distance, so the image’s appearance can change.
| Part | Its job in the device | Observation to make |
|---|---|---|
| Pinhole | Allows a narrow set of paths from the scene into the box. | Keep it small and point it at a bright scene. |
| Tracing-paper screen | Receives the light that forms the image. | Look for the scene’s colours and inverted outline. |
| Sliding inner box | Changes the gap between opening and screen. | Move it until the image is easiest to see. |
| Dark viewing cloth | Reduces distracting outside light. | Compare visibility with and without it. |
Problem
A student can see a pen image in a plane mirror and a tree image on the tracing paper of a pinhole camera. Explain the difference.
- 1.In the camera, light from the tree passes through a hole and reaches a real screen; the picture there is inverted.
- 2.In the mirror, reflected light reaches the eye and the pen appears behind the mirror; moving a screen does not capture that image.
- 3.Both are called images, but their formation and the screen observation differ.
Compare the Two Images Carefully
A plane mirror makes an erect, same-size image with perceived left-right reversal; the image is not obtained on a screen. A pinhole camera makes an upside-down image on its screen. Seeing colour in a pinhole image and seeing a dark shadow are also different observations. Use a labelled sketch of the source, opening or mirror, and screen or eye to decide which explanation fits.
Quiz
What part of a pinhole camera receives its image?
Where does light from the top of an object reach the pinhole-camera screen?
Why is the pinhole image inverted?
Which image can be obtained on a screen in the activities?
What material forms the viewing screen in the sliding-box camera?
What should a student do to see an outdoor pinhole image more clearly?
Practice Problems
- Sketch a candle, pinhole, and screen. Trace paths from the top and bottom to predict the image orientation.
- Explain why the shadow of a candle-shaped cutout and the pinhole image of a lit candle are not the same kind of observation.
- Name the job of the pinhole, tracing-paper screen, and sliding inner box.
- A student sees the top of a tree at the top of a camera screen. Explain what the student should check in the drawing.
- Compare a mirror image and a pinhole image for uprightness, screen capture, and apparent location.
- Propose a safe outdoor observation using the sliding-box camera and state what you would record.
Key Takeaways
• A small opening allows light from an object to form an image on a screen. • Straight paths from top and bottom cross at the pinhole, producing an inverted image. • The sliding-box camera uses a pinhole, a tracing-paper screen, and a dark viewing area. • A plane-mirror image appears erect and cannot be obtained on a screen; a pinhole image can.