An autumn lantern starts with a dry "pop" as a pencil pushes through paper. Making a lantern with kids takes half an hour and needs only an empty jam jar, a sheet of dark paper and a battery tealight. In a dark room, the lantern draws a pattern of dots on the wall. But slide it a few steps back and something odd happens. The dots grow. The holes in the paper are exactly as small as before. So where does the extra size come from?
What happens when light passes through a hole?
Light travels in straight lines. The tealight sends light out in every direction, but the dark paper stops almost all of it. Only where there is a hole does a little light escape, and it carries straight on until it hits the wall.
The light from one hole spreads out like a cone, a bit like a torch beam. Right up against the wall, the light has almost no room to spread, so the dot is as small and sharp as the hole. From the middle of the room, the cone has had plenty of time to widen.
The holes don't get bigger when you move the lantern. The light just gets room to spread.
The big dots are also fainter. The same amount of light gets through each hole wherever the lantern stands, but when it has to fill a bigger patch, every part of that patch gets dimmer. It is like spreading one spoonful of jam over a slice of bread, or over the whole kitchen table. More autumn experiments are collected in Why is autumn the best laboratory a child can have?
Try it at home: the lantern that draws on the wall
Suitable for: 4–12 years. ⚠️ An adult helps with the smallest holes. Use a battery tealight, never a real flame, because paper can catch fire.
Before you switch it on, ask: if light travels in straight lines, will the pattern get bigger, smaller or stay the same when the lantern moves away from the wall? Write down your guess.
- Cut the paper to wrap around the jar and lay it on an old towel.
- Draw a pattern of dots, such as the Plough or your first initial.
- Poke the pencil through each dot, some deep and some barely.
- Tape the paper around the jar and put the lit tealight inside.
- Turn off the main light and set the lantern right against a pale wall.
- Slide it slowly backwards and watch the dots.
The same straight-line light makes shadows outdoors, which you can explore in Draw the light: how shadows change as winter comes. And to find out why some things send light straight back towards a lamp, read Why do reflectors shine?
What happens if you poke two holes right next to each other and slide the lantern all the way to the other side of the room?
Questions to wonder about
- Which pattern would you poke if your lantern could only shine for one evening?
- Why do you think the smallest dots are the first to disappear when the lantern is far away?
- The stars in the sky are dots of light too. Are they small because they are small, or because they are so far away?
Every child is made of good atoms
One dark evening the lantern sits on the windowsill, scattering dots of light across the curtain.
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