DIY Periscope: Seeing Around Corners with Simple Reflection Physics

Can You Really Build a Working Periscope That Lets You See Around Corners?

Yes, and it's one of the most rewarding maker projects you can tackle with middle-school and older kids. A DIY periscope uses two small mirrors positioned at precise 45-degree angles inside a tube to bounce light from an object into your line of sight. Unlike many quick experiments that get disassembled right after, a well-built periscope becomes a tool your kids will actually keep, use for backyard spy games, and proudly show friends. The construction takes anywhere from thirty minutes to an hour depending on your materials and precision, but the physics lesson: and the fun: lasts much longer.

This project sits at the sweet spot where simple optical principles meet hands-on engineering. You're teaching reflection, angles, and line-of-sight concepts while building something genuinely useful. Submarines use periscopes to see above water while staying submerged. Your kids can use theirs to peek over fences, around doorways, or even check what's happening on a high shelf without climbing.

Why Two Mirrors Bend Your Line of Sight

The secret to any periscope is double reflection. When light bounces off a mirror, it follows the law of reflection: the angle at which light hits the mirror equals the angle at which it bounces away. If you position a mirror at 45 degrees to your line of sight, it redirects light at a 90-degree turn: essentially sending your view straight up (or down, depending on which mirror).

In a periscope, the first mirror (at the top) catches light from the object you want to see and reflects it downward through the tube. The second mirror (at the bottom, near your eye) catches that reflected light and bounces it horizontally into your eye. Because both mirrors sit at matching 45-degree angles and face opposite directions, they work together to bend your vision around obstacles or over barriers.

This isn't magic: it's geometry. The two reflections add up to redirect your view by 180 degrees of travel but maintain the upright orientation of the image. That's why submarine crews could navigate using periscopes without their view being upside down or reversed.

What You'll Need: Materials and Cost Breakdown

Building a functional periscope requires surprisingly few materials, and most can be sourced from around the house or a single hardware store trip.

Material	Quantity	Estimated Cost	Notes
Cardboard (poster board or flat cereal box)	2 large sheets	$2–5	Thicker poster board holds shape better
Small mirrors (2" × 2" or similar)	2	$3–8	Craft mirrors, old compact mirrors, or cut mirror tiles
Clear packing tape or duct tape	1 roll	$3–5	For assembly and securing mirrors
Ruler or measuring tape	1	$1–3	Precision matters for angles
Scissors or craft knife	1	$2–5	Sharp blade helps clean cuts
Protractor (optional but helpful)	1	$1–2	Ensures accurate 45-degree angles
Pencil	1	$0.50	For marking cut lines
Total	:	$12–28	Reusable tools bring cost down over time

Alternative Materials:

• Two empty rectangular tissue boxes (already tube-shaped)
• PVC pipe sections with elbow joints (more durable, higher cost)
• Old CDs scored and broken into four pieces each (budget mirror substitute)
• Milk carton panels (lightweight, easy to cut)

The beauty of this project is adaptability. A quick-and-dirty version using cereal boxes and CD fragments can cost under five dollars. A sturdier version with purchased mirrors and thick poster board might run closer to twenty dollars but will withstand repeated use.

Step-by-Step: Building Your Periscope

Step 1: Measure and Cut Your Tube Sections

Start by deciding your periscope's height. A good working length for kids is around 12 to 18 inches: tall enough to peek over furniture or around corners without becoming unwieldy.

Cut two rectangles from your cardboard, each about 12–18 inches long and 6–8 inches wide. These will form the outer walls of your periscope tube. Roll one rectangle into a square or rectangular tube shape, overlapping the edges by about half an inch, and tape securely along the seam. This creates your main viewing channel.

If you prefer a nested double-wall design for extra strength, roll the second rectangle into a slightly smaller tube that fits snugly inside the first. This approach reduces wobble and makes the periscope more durable for outdoor use.

Step 2: Create Mirror Mounts with Precise Angles

This step determines whether your periscope works or becomes a frustrating cardboard tube. You need to position each mirror at exactly 45 degrees to the tube's axis.

Cut four identical right triangles from cardboard scraps. Each triangle should have two equal sides (forming the 90-degree corner) of about 2–3 inches, with the longest side (the hypotenuse) matching the width of your mirrors. These triangles become mounting brackets.

Use a protractor to verify the angles if you have one. The two base angles should each measure 45 degrees. If you're eyeballing it, fold a square of paper diagonally: the crease creates a perfect 45-degree angle you can trace.

Step 3: Attach Mirrors to Triangle Mounts

Tape each mirror to the hypotenuse (longest edge) of one triangle. The reflective surface should face inward, toward the triangle's 90-degree corner. You'll need two of these mirror-triangle assemblies: one for the top of your periscope, one for the bottom.

Test the angle by holding the assembly up to a light source. When positioned at 45 degrees to vertical, the mirror should reflect the light at a 90-degree turn. This is your checkpoint before permanently installing the mirrors.

Step 4: Install Mirrors Inside the Tube

Slide the first mirror assembly into the top of your tube with the mirror facing downward at a 45-degree angle. The mirror should sit near one edge of the tube's opening, creating a diagonal barrier across part of the tube.

Secure it firmly with tape on all edges. The triangle's two equal sides should rest flush against the tube walls, with the mirror angled across the corner.

Now install the second mirror at the bottom of the tube, but facing upward at 45 degrees: opposite direction from the first mirror. This mirror should sit near the opposite edge from the top mirror, maintaining symmetry.

Step 5: Cut Viewing Windows

Mark and cut a rectangular viewing window at the top of the tube, positioned so you can see into the top mirror. This is where light enters your periscope from the object you're observing.

Cut a second viewing window at the bottom of the tube, near the bottom mirror, positioned so you can look through it into the mirror. This is your eyepiece.

The windows should be large enough for comfortable viewing but not so large they weaken the tube's structure. A 2-inch × 3-inch window typically works well.

Step 6: Test and Adjust

Look through the bottom eyepiece while pointing the top window at an object across the room. You should see a clear reflection of that object in your view. If the image appears cut off, dim, or missing entirely, one of your mirrors likely needs adjustment.

Common fixes:

• Mirror angle too steep or shallow: loosen tape, adjust triangle position, re-secure
• Mirrors not parallel: ensure both triangles have identical angles
• Windows blocked: trim away any cardboard obstructing the light path

Once you achieve a clear view, reinforce all tape joints and mirror mounts. Your periscope is ready for action.

Making It Last: Durability Tips and Decorative Options

Since this project emphasizes creating a keeper tool rather than a one-time demo, consider these enhancements:

Weatherproofing: Wrap the exterior in clear packing tape or contact paper to protect against moisture if kids plan outdoor spy missions.

Decoration: Let kids personalize the tube with paint, stickers, or camouflage patterns before final assembly. Keep decorations away from the mirror areas and viewing windows.

Reinforcement: Add a strip of duct tape along all seams and corners for extra strength. This prevents crushing if the periscope gets tossed in a toy bin.

Handle or Strap: Attach a cardboard handle or fabric strap to make the periscope easier to carry and position during use.

Physics Extensions: What Else Can You Explore?

Once your periscope works, use it as a springboard for related optical experiments:

Mirror Quality Comparison: Build two identical periscopes, one with high-quality glass mirrors and one with lower-quality reflective material (aluminum foil, polished metal). Compare image clarity and brightness. Which material reflects more efficiently?

Angle Experimentation: What happens if you adjust the mirrors to 30 degrees or 60 degrees instead of 45? Document how different angles affect the viewing direction and image orientation.

Extended Periscope: Can you chain together multiple mirror reflections to create an even taller periscope or one that bends around multiple corners? This gets complex quickly but offers strong geometry practice.

Field of View Study: Measure how much of a room you can see through the periscope compared to direct viewing. Calculate the field-of-view angle using basic trigonometry.

Real-World Applications Worth Discussing

Periscopes aren't just toys. They appear in contexts that might surprise kids:

• Submarines: Military and research submarines use sophisticated periscopes with multiple lenses and prisms to observe surface conditions while remaining submerged
• Medical endoscopes: Doctors use similar reflection principles in endoscopes to see inside the body during minimally invasive procedures
• Trench warfare: Soldiers in World War I used periscope rifles to aim and fire from protected positions
• Construction and inspection: Workers use periscopes to inspect cramped or dangerous spaces without entering them
• Theater and stage design: Backstage crews sometimes use periscopes to monitor stage action from hidden positions

These applications help frame the project as more than craft time: it's introductory engineering with tangible connections to real professions.

Frequently Asked Questions

Can I use plastic mirrors instead of glass?

Definitely. Plastic craft mirrors work well for this project and offer the safety advantage of being shatter-resistant. The trade-off is slightly lower optical quality: plastic tends to produce dimmer, less sharp reflections than glass: but for a DIY learning tool, the difference is minimal.

Why does my periscope show an upside-down image?

This typically means one or both mirrors are installed at the wrong angle or facing the wrong direction. Double-check that the top mirror faces downward and the bottom mirror faces upward, with both at matching 45-degree angles. If one mirror is reversed, the second reflection fails to correct the image orientation.

How do I make a periscope that turns corners horizontally instead of vertically?

The same principle applies, but you'll position your mirrors to redirect light sideways rather than up-and-down. Mount both mirrors on opposite vertical walls of the tube instead of the top and bottom. This creates a horizontal periscope useful for seeing around doorways or obstacles at eye level.

Can I use curved mirrors or lenses to magnify the view?

Standard periscopes use flat mirrors to maintain true-size images. Adding curved mirrors or magnifying lenses changes the design significantly and typically requires precise positioning and focal length calculations. If you want to experiment with magnification, consider building the basic periscope first, then adding a small magnifying lens as an eyepiece attachment.

Why is the image dimmer than direct viewing?

Each reflection loses a small percentage of light due to absorption and scattering. Since periscopes use two reflections, you'll naturally see some brightness reduction compared to direct line of sight. Higher-quality mirrors with better reflective coatings minimize this loss. Very long periscopes with many reflections can become quite dim.

How can I seal the tube to keep dust off the mirrors?

Cover the top and bottom windows with clear plastic wrap or acetate sheets held in place with tape around the edges. This keeps dust and fingerprints off the mirrors while maintaining visibility. Just be sure the plastic stays taut to avoid distortion.

What Makes This Project a Winner

Unlike many kitchen-table science demonstrations that end in the recycling bin, a well-constructed periscope earns shelf space. Kids use them for imaginative play, older students use them to understand optical geometry, and adults appreciate the elegant simplicity of the physics at work.

The maker aspect matters here. Building something functional from raw materials builds confidence and problem-solving skills in ways that pre-packaged kits can't match. When kids face a mirror-angle problem and work through the solution, they're doing real engineering: just at a comfortable scale with safe, accessible materials.

The reflection physics you demonstrate here lays groundwork for understanding more complex optical systems later. Cameras, telescopes, microscopes, fiber optics, and laser systems all rely on controlled light paths using mirrors and lenses. Your DIY periscope might seem simple, but it introduces principles that underpin billion-dollar technologies.

Plus, let's be honest: any project that lets kids spy around corners is going to hold their attention better than most textbook explanations of reflection angles ever could.

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Disclaimer: This article provides educational guidance for a hands-on science project intended for middle-school-aged children and older under adult supervision. While periscope construction uses generally safe materials, adult oversight is recommended during cutting and assembly phases. Mirrors: whether glass or plastic: should be handled carefully to avoid cuts or breakage. This project is designed for learning purposes and should not be used in any context where safety, privacy, or security might be compromised. The techniques and information shared here are based on established optical principles and common DIY practices, but individual results may vary based on materials, construction precision, and environmental factors. Tierney Family Farms and its contributors are not responsible for injuries, property damage, or other issues that may arise from attempting this project. Always follow local regulations regarding privacy and observation when using completed periscopes.