The pinhole camera: light travels in straight lines
Point a pinhole camera at a candle. Move the candle, make the box longer and the hole bigger, and see how the upside-down image on the screen changes.
Core concept
Light travels in straight lines, so rays from a candle cross at the pinhole and make an upside-down image on the screen
Topic: Light
A pinhole camera is just a closed box with a tiny hole in the front and a thin screen (tracing paper) at the back. Light from a candle travels in straight lines. Light from the top of the flame passes through the hole and carries on downwards to the bottom of the screen; light from the bottom of the candle goes up to the top of the screen. The rays cross at the hole, so the image is upside down. Its size depends on how far the candle and the screen are from the hole, and the size of the hole decides how bright and how sharp it is.
What you will learn
- Explain how a pinhole camera shows that light travels in straight lines.
- Explain why the image in a pinhole camera is upside down.
- Predict how the image changes when the candle moves or the box gets longer.
- Describe how the size of the hole changes the brightness and sharpness of the image.
Before you start
- Know that we see things when light from them enters our eyes.
Materials
- A candle and a matchbox
Equipment
- A pinhole camera: a box (or a tube) with a pinhole at one end and tracing paper at the other
Procedure
- Record the image of the candle 100 cm from a 20 cm box with a small hole.
- Move the candle to 50 cm and record.
- Put the candle back at 100 cm, make the box 30 cm long and record.
- Make the hole bigger and record.
How this simulation is simplified
- The candle (with its flame) is 20 cm tall.
- The small hole is 1 mm wide and the big hole 5 mm wide; the blur is how wide the hole's patch of light is on the screen.
- Brightness is shown as a percentage of the brightest image here (big hole, 10 cm box).