Have you ever wanted to peek over a tall fence, spy around a corner, or watch what’s happening on the other side of the sofa without anyone seeing you? Submarine sailors solve that exact problem deep under the sea — and you can build their secret tool at home using two mirrors and a cardboard box. Let’s make a working periscope!
🧪 What You Need
- A long cardboard tube or box (a milk carton works well)
- Two small flat mirrors the same size
- Scissors (ask an adult)
- Sticky tape
- A pencil and ruler
Let’s Build It — Step by Step
⚠️ Stay Safe
Mirrors and scissors have sharp edges. Let a grown-up do the cutting, and tape over any glass corners so no one gets a nasty scratch.
What’s Going On?
Light usually travels in straight lines, so normally you can only see things that are right in front of your eyes. A mirror changes that. When light hits a flat mirror, it bounces off in a new direction — this is called reflection.
Your periscope uses this trick twice. Light from the top of the room hits the upper mirror and bounces straight down the tube. Then it hits the lower mirror and bounces again, this time straight into your eye. Two bounces carry the picture all the way down to you, which is how the periscope lets you see over walls and around corners.
📖 Big Word: Reflection
When light bounces off a smooth, shiny surface and travels off in a new direction — just like a ball bouncing off a wall.
🌟 Did You Know?
Real submarines use periscopes too, so the crew can peek above the waves while the boat stays hidden under the sea. Many modern subs now use cameras and screens instead of mirrors!
The Law of Reflection: Bounce In, Bounce Out
Mirrors follow one beautifully simple rule: light bounces off at the same angle it arrives — scientists call it the “angle in equals the angle out.” It’s just like a ball bouncing off a wall! That’s the whole secret to why your periscope’s mirrors are tilted at 45 degrees (halfway to flat). Light coming in level hits the top mirror and turns a perfect right-angle straight down the tube, then the bottom 45° mirror turns it another right-angle straight into your eye. Tilt a mirror the wrong way and the light bounces off somewhere else — and you see nothing! That’s why taping them at just the right angle matters so much.
📖 Big Word: Ray
A ray is a single straight line of light travelling from one place to another. Light always moves in straight-line rays — until a mirror bounces a ray off in a brand-new straight line.
Mirror Mystery: Why Words Look Backwards
Hold a book up to a mirror and the writing reads backwards — but here’s the twist: a flat mirror doesn’t really swap left and right. It actually swaps front and back (what’s near you looks far away in the mirror). Our clever brains just read that as a left-right flip. There’s a brilliant real-world use for this: look at the front of an ambulance and you’ll see “ECNALUBMA” written backwards — so when a driver ahead spots it in their rear-view mirror, it flips the right way round and they know to pull over!
Bouncing Light Is Everywhere
Once you start looking, reflection is everywhere. You can see your own face because it bounces light into the mirror. The Moon glows only because it reflects the Sun’s light — it makes no light of its own! Kaleidoscopes, a dentist’s little angled mirror, and even the internet use bouncing light — the signals in fibre-optic cables race along by reflecting off the inside walls, all the way across the world. And if you stand between two mirrors facing each other, you’ll see a tunnel of “forever” reflections bouncing back and forth!
🧠 Quick Quiz — tap an answer!
1. What does light do when it hits a flat mirror?
2. How many mirrors does a periscope need to work?
3. Who uses periscopes to see above the water?
Frequently Asked Questions
Q: How does a periscope let you see around corners?
Two tilted mirrors bounce the light twice: the top mirror sends the picture down the tube, and the bottom mirror sends it into your eye — so you see over walls and around corners.
Q: Why are the mirrors tilted at 45 degrees?
Because light bounces off at the same angle it hits (the law of reflection). A 45° mirror turns level light a perfect right-angle down the tube, and the second one turns it into your eye.
Q: Why does writing look backwards in a mirror?
A mirror swaps front and back, not really left and right — our brains read that as reversed text. It’s why “AMBULANCE” is printed backwards, so it reads correctly in a driver’s mirror.
Q: Does the Moon make its own light?
No — moonlight is really sunlight bouncing (reflecting) off the Moon back to us.
Q: Where else is reflection used?
In mirrors, kaleidoscopes, dentists’ mirrors, and fibre-optic cables that carry the internet by bouncing light along inside them.
📚 Where we learned this (for curious grown-ups):
• Britannica — The Law of Reflection
• Exploratorium — Mirror Reversal (Why Words Look Backwards)
• NASA — Moonlight Is Reflected Sunlight
You Did It! 🎉
You just built a real working periscope and bent light around a corner — super spy work! Now try changing one thing: what happens if you make the tube longer, or tilt the mirrors at a slightly different angle? See if you can still spot around the corner, and discover how the bouncing light changes!
