Periscope Experiment for Kids: Mirrors That Aim Light

Hold a page of writing in front of a single mirror and the words come out backwards. Hold the same page in front of a periscope with two mirrors and the words read perfectly normally. That contrast is the most useful thing in this periscope experiment for kids, because it quietly demolishes one of the most repeated wrong statements in everyday physics: that mirrors reverse left and right. They do not. They reverse front and back, and a periscope proves it in about four seconds. Building one takes half an hour, and there is a single construction detail that decides whether it works or produces a tilted, useless view.

Key Takeaways

• You mount two mirrors at 45° in a long box so that you can see over a wall, around a corner, or above a crowd.
• The construction trick: cut matching 45° slots in both side walls so the mirrors slide into place parallel to each other. Gluing them by eye is the main cause of failure.
• The field of view is narrow, roughly 10° for a 5 cm mirror in a 30 cm tube, which is honest to say up front and is why real periscopes use lenses.
• Text viewed through two mirrors reads normally, which is the evidence that a mirror does not swap left and right.
• Best for ages 7 to 13, with an adult doing the cutting and handling any glass.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Two flat mirrors, 5 × 5 cm or larger	2	Craft mirror tiles, or acrylic mirror sheet cut to size	Two mirrored compacts, or two pieces of highly polished steel, which give a dimmer view
Long rigid box or tube	30–40 cm	Two 1 L drinks cartons taped end to end; a long thin gift box	Roll two sheets of corrugated card into a square tube and tape it
Craft knife and cutting mat	1	n/a	Adult only. A box cutter and a chopping board you do not mind marking
Ruler and protractor	1 each	n/a	A square of paper folded corner to corner gives an exact 45°
Strong tape	1 roll	n/a	Masking tape for assembly, then duct tape for strength
Black paint or black paper	Enough to line the inside	n/a	A black marker on the inner walls reduces stray reflections
Printed page of text	1	n/a	For the left-right reversal test
Metre ruler	1	n/a	For the field-of-view measurement

Time: prep 30 min, experiment 20 min, cleanup 5 min Cost: two craft mirrors cost about the same as a coffee; everything else is scrap Mess level: low, though cutting card makes a mess of a table

Do the Experiment: A Periscope Experiment for Kids

Step 1 - Build the tube and line it black

Tape two drinks cartons end to end to make a tube 35 to 40 cm long, or use a long box of similar size. Paint or paper the inside black. Stray light bouncing off pale cardboard walls is the difference between a clear view and a washed-out one, and almost no tutorial mentions it.

Step 2 - Cut matching 45° slots, not mirror mounts

This is the step that matters. At each end, mark a line across both side walls at exactly 45° to the tube’s axis, with the two lines sloping the same way, and cut a narrow slot along each. The mirrors then slide in and sit parallel to each other by construction, not by your judgement. Use a square of paper folded corner to corner as your 45° gauge.

Step 3 - Cut the viewing windows

Cut a square window in the front face at the top end and a matching one in the back face at the bottom end, each the same size as your mirrors and positioned so the mirror faces it. Check by eye before taping anything.

Step 4 - Slide in the mirrors and predict

Slide a mirror into each slot, reflective side facing into the tube, and tape them firmly. Then ask three questions before looking: “Will the view be upright or upside down? Will writing read normally or backwards? How much will I be able to see?” Record all three answers.

What you should see: Look in the bottom window and you see whatever is in front of the top window, as if your eye were up there. The view is upright. If it is tilted or shows the inside of the tube, your slots are not at matching angles.

Step 5 - Test the writing

Hold a printed page in front of the top window and read it through the periscope. Then hold the same page in front of a single flat mirror.

What you should see: Through the periscope, the text reads normally, left to right, exactly as on the page. In the single mirror it is reversed. Two reflections undo what one does, and that is the whole proof.

Step 6 - Measure the field of view

Have someone hold a metre ruler 2 m in front of the top window, across your line of sight. Record how many centimetres of it you can see at once.

What you should see: Around 30 to 40 cm of ruler visible at 2 m with 5 cm mirrors in a 35 cm tube, which works out to roughly a 10° field. For comparison, your unaided comfortable field is more than 60°. The periscope view is like looking through a short straw, and knowing that in advance stops it being a disappointment.

Step 7 - Rebuild it as a corner-looker

Re-tape the whole thing so the two windows face sideways in opposite directions rather than up and down.

What you should see: The same physics lets you see around a corner while standing behind a wall. The geometry does not care which way is up, which is a useful thing for a child to notice on their own.

What’s Actually Going On

Light reflecting off a smooth surface obeys one rule: the angle of incidence equals the angle of reflection, both measured from the normal, the line perpendicular to the surface. A mirror tilted at 45° to a horizontal ray has its normal at 45° too, so the ray arrives 45° from the normal and leaves 45° on the other side, which turns a horizontal ray into a vertical one. Do that twice and the light is travelling horizontally again, but displaced.

The interesting part is what happens to the image, and it is cleaner than most people expect. A single plane mirror performs a reflection, which is a transformation that reverses handedness. Two reflections in parallel planes compose into a pure translation: the image is shifted in space and nothing else is changed. It is not rotated, not flipped and not reversed. That is why text reads normally through a periscope, and it is a mathematical fact you can verify with a page of print.

Which brings us to the misconception. “A mirror reverses left and right” is false as stated. What a plane mirror actually reverses is the direction perpendicular to its surface: front becomes back. When you face a mirror and raise your right hand, the image’s hand on your right side goes up too; nothing has swapped sides. The confusion comes from how we describe the image. To compare yourself with your reflection we imagine walking round behind the mirror and turning to face the other way, and it is our mental rotation that introduces the left-right swap. Yohtaro Takano analysed this in Psychonomic Bulletin & Review in 1998 and argued that several distinct processes contribute to the impression of left-right reversal, depending on whether you are reasoning about the object, your own body, or the shape of the letters. Lie a mirror flat on the floor and the reflection looks vertically flipped instead, which is the same effect with a different axis.

Two more honest details about the hardware. First, the field of view really is tiny, set by the ratio of mirror width to tube length. A 5 cm mirror 30 cm from your eye subtends about 9.5°, and you cannot do better without larger mirrors, a shorter tube, or lenses. Submarine and tank periscopes use multi-element optics with wide-angle objectives precisely to escape this limit, which is a nice reason to care about lenses.

Second, ordinary household mirrors are back-silvered: the reflective coating sits behind a few millimetres of glass, so you get a faint secondary reflection from the front glass surface. In a two-mirror system that gives a soft ghost image offset from the main one. Front-surface acrylic mirror, the kind sold for craft and model making, has no ghost at all and is both safer and optically better. If you want a reason to spend a few pounds on this project, that is it.

How to Explain It to Your Kid

Ages 5–8: The mirror throws the light round the corner

Shine a torch at a mirror held at an angle and let them watch the beam change direction on the wall. Let them aim it deliberately: at the ceiling, at a doorway, at a teddy. Then tell them the periscope is two of those stacked up, one to throw the light down and one to throw it into their eye. Then let them use it to spy over the back of the sofa, which is the entire reward and is enough at this age.

Ages 9–12: Equal angles, and two mirrors undo one

Draw the ray diagram with the normal marked and the two equal angles labelled. Then do the writing test and introduce the result: one mirror flips, two mirrors do not. Have them predict what three mirrors would do before testing it with a hand mirror added to the path. This connects naturally with the straight-line behaviour of light in our collection of optics experiments with a flashlight and cardboard.

Ages 13+: Transformations and the handedness argument

Give them the formal version: a plane reflection has determinant −1 and reverses orientation, so an even number of reflections gives determinant +1 and preserves it. Two parallel reflections compose to a translation by twice the plane separation. Then ask them to work out what two mirrors at 90° to each other do, which is a 180° rotation about their line of intersection, and why a corner reflector of three mutually perpendicular mirrors sends light straight back where it came from. That last one is how the retroreflectors left on the Moon work.

The question to ask: “If I put a third mirror into the path, would the writing read normally or backwards? Say which, and say what rule you used, before we test it.”

Why Teaching This Early Pays Off

Reflection is the first topic in optics where a child can make a confident prediction, test it in seconds and be wrong. That makes it an unusually good place to practise changing your mind on evidence. The left-right claim is not a minor technicality either: it is repeated in children’s books, in science shows and by most adults, which means a child who has tested it has had the experience of finding that a widely believed statement does not survive contact with a page of print.

The geometry transfers directly. Equal angles about the normal is the same rule that governs how a cue ball leaves a cushion, how a solar reflector concentrates light, how a sound echo returns and how a bicycle reflector sends a car’s headlight back to the driver. Galili and Hazan’s 2000 work on learners’ knowledge in optics describes how fragmentary students’ models of light remain even after instruction, and the fix their analysis implies is not more explanation but more cases in which a prediction is made and then checked.

One caution on expectation. A periscope is a toy with a terrible field of view, and children who have seen submarine films expect a cinematic view. Say so beforehand. Framing the narrow field as a measurement to be made, rather than a flaw to be discovered, turns the weakest part of the build into the most interesting one.

Before You Start: What Parents Should Know

Best age	7 to 13. Under 7, let an adult build it and let the child use it
Supervision	Adult required for all cutting and for handling any glass mirror
Real hazards	Glass mirrors chip and the shards are very sharp. Use acrylic mirror sheet if you can; if you use glass, tape every edge with duct tape before it goes near a child. Craft knives are for adults only. Never use the periscope to reflect sunlight toward anyone’s face, and never look at the sun through it
Don’t	Don’t substitute a broken piece of a household mirror, however convenient the size looks. The cut edges of a fractured mirror are the single most likely injury in this whole batch of experiments

When It Doesn’t Work

• You see the inside of the tube. The mirrors are not at 45°, or the windows are not aligned with them. Re-check your slots against a folded-paper 45° gauge and make sure each window faces its mirror squarely.
• The view is tilted. The two slots are not cut at matching angles, or one mirror is rotated in its slot. This is why slots beat glue: a glued mirror can rotate a few degrees without you noticing, and the tilt doubles across two reflections.
• Everything looks grey and washed out. Stray light. Line the inside with black paper and make sure the only openings are the two windows. Pale cardboard interiors scatter light badly.
• You see two overlapping images. You are using back-silvered household mirrors and seeing the ghost reflection from the front glass surface. Front-surface acrylic mirror removes it entirely.

Take It Further

• Measure field of view against mirror size. Build the same tube with 3 cm, 5 cm and 8 cm mirrors and measure the visible width of a metre ruler at 2 m each time. Plot visible width against mirror width; it should be close to a straight line through the origin, and the slope lets you predict any combination.
• Make a 90° corner reflector. Tape three small mirrors into the corner of a box so they are mutually perpendicular, then shine a torch in from any angle. The beam comes straight back at you regardless of how you aim it. That is the principle behind road studs and the lunar laser-ranging retroreflectors.
• Chain it with refraction. Replace one mirror with a water-filled container and look at the bend instead, as in the broken pencil refraction experiment, then try guiding light along a stream using total internal reflection. Three different ways of steering light, one afternoon.

What to Watch For Over the Next 3 Months

• Week 4: They correct someone who says mirrors swap left and right. Expect this to cause an argument, which is a good sign.
• Month 2: They notice the angled mirror in a car’s wing mirror, a dentist’s mirror or a shop security mirror and can say what it is doing.
• Month 3: If they ask why the ambulance lettering on the front of the vehicle is written backwards, they have joined the two halves: it is mirrored so it reads correctly in your rear-view mirror, which is a single reflection and therefore reversed.

Frequently Asked Questions

Why does writing read normally through a periscope but backwards in a mirror?

Because each reflection reverses handedness and two reflections cancel. Mathematically, reflections in two parallel planes compose into a simple translation, so the image is displaced in space but otherwise unchanged. One mirror reverses, two do not.

Do mirrors really not reverse left and right?

Not as commonly stated. A plane mirror reverses the axis perpendicular to its surface, so front becomes back. The left-right description comes from mentally rotating yourself to compare with the image. Lay the mirror on the floor and the same effect looks like an up-down flip instead.

Why is the view so narrow?

Because the field of view is set by the mirror width divided by the distance from your eye. A 5 cm mirror 30 cm away subtends about 9.5°. Bigger mirrors or a shorter tube widen it; real periscopes use lens systems to do far better.

Should I use glass or acrylic mirrors?

Acrylic, for two reasons. It will not shatter, and because the coating is on the front surface it produces no ghost image. Household glass mirrors are back-silvered, so you see a faint second reflection from the glass surface in front of the coating.

Can two mirrors be at angles other than 45°?

Yes, as long as they are parallel to each other, which keeps the composite transformation a translation. Shallower angles make a longer, thinner offset; steeper ones a shorter, fatter one. Non-parallel mirrors rotate the image, which is sometimes useful and usually confusing.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. The Physics Classroom. “The Law of Reflection.” https://www.physicsclassroom.com/class/refln/Lesson-1/The-Law-of-Reflection
2. The Physics Classroom. “Image Characteristics for Plane Mirrors.” https://www.physicsclassroom.com/class/refln/Lesson-2/Image-Characteristics
3. Takano, Y. (1998). “Why does a mirror image look left-right reversed? A hypothesis of multiple processes.” Psychonomic Bulletin & Review, 5(1), 37–55. https://doi.org/10.3758/BF03209456
4. Galili, I., & Hazan, A. (2000). “Learners’ knowledge in optics: interpretation, structure and analysis.” International Journal of Science Education, 22(1), 57–88. https://doi.org/10.1080/095006900290000
5. Nelson, J. B., & Nelson, J. (2020). “Activity 4: Shadows and Straight-Line Propagation of Light.” In Teaching About Geometric Optics. AIP Publishing. https://doi.org/10.1063/9780735422179_004
6. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
7. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165