The DIY Periscope: Reflection and Angles (#97)
Experiment at a Glance
Recommended Age: 8-14 years
Cost: Under $10
Difficulty: Intermediate
Time Required: 40 minutes
Learning Focus: Light reflection, angles, optical physics
What Is a Periscope and How Does It Let You See Around Corners?
A periscope is an optical device that uses two mirrors positioned at precise 45-degree angles to redirect light, allowing you to see over walls, around corners, or above crowds without being directly in line with what you're viewing. The secret lies in how light bounces, when it hits a mirror at a specific angle, it reflects away at that exact same angle, creating a predictable path you can control.
In this hands-on experiment, you'll build a working periscope from cardboard and small mirrors, then discover why that 45-degree angle isn't just a suggestion, it's the mathematical sweet spot that makes the whole system work.
Why Do Periscopes Use 45-Degree Mirrors?
Here's where the physics gets fun. When light enters your periscope through the top opening, it hits the first mirror tilted at 45 degrees. The law of reflection states that light bounces off a surface at the same angle it arrives, so if it comes in at 45 degrees, it leaves at 45 degrees too.
That first bounce sends the light straight down the tube at a 90-degree turn. When it reaches the bottom mirror (also at 45 degrees but facing the opposite direction), it gets redirected again at another 90-degree angle, shooting straight out toward your eye.
If the mirrors were tilted differently, say, at 30 degrees or 60 degrees, the light would scatter into the walls of the tube instead of reaching your viewing hole. The 45-degree angle creates two perfect right-angle turns, keeping the light on track from entrance to exit. It's geometry working in your favor.
What You'll Need to Build Your Periscope
Gathering materials is half the fun. Here's your shopping list:
Structural Materials:
- Two empty milk cartons (half-gallon size works great) or one long cardboard tube
- Strong tape (duct tape or packing tape)
- Scissors or a craft knife
- Ruler and pencil
Optical Components:
- Two small square mirrors (approximately 2-3 inches wide, check craft stores or dollar stores)
- Optional: small pocket mirrors work if you can't find loose mirror tiles
Safety Gear:
- Adult supervision for cutting
- Gloves if handling glass mirrors with sharp edges
Pro tip: If you're having trouble sourcing inexpensive mirrors, craft stores often sell small mirror tiles in packs, or you can salvage mirrors from old cosmetic compacts.
Step-by-Step: Building Your Periscope
Step 1: Prepare Your Viewing Tube
If you're using milk cartons, rinse them thoroughly and let them dry. Cut off the top and bottom of each carton so you have two open-ended rectangular tubes. If you're using a single cardboard tube (like from paper towels or poster shipping tubes), you're already set.
Measure the interior dimensions of your tube, you need to know how wide it is so your mirrors fit snugly inside later.
Step 2: Mark Your Mirror Slots
On one carton, measure up about 2 inches from the bottom. Draw a diagonal line from the bottom-left corner to a point 2 inches up on the right side (or vice versa, depending on which direction you want). This creates a 45-degree angle. Cut carefully along this line to create a slot where your mirror will slide in.
Critical detail: The slot should be angled so the mirror faces upward and outward, you want the reflective surface pointing toward the top opening of the tube.
Repeat this process on the second carton, but this time position the slot near the top (about 2 inches down from the top edge), and angle it in the opposite direction. This mirror needs to face downward and outward toward the viewing hole you'll cut next.
Step 3: Cut Viewing Holes
On the first carton (with the lower mirror slot), cut a rectangular viewing hole on the side just below where your mirror will sit. This is where your eye will look through.
On the second carton (with the upper mirror slot), cut a similar rectangular hole on the opposite side, just above where that mirror will sit. This is where light enters the periscope.
The holes should be on opposite sides of the tube so light has to bounce off both mirrors to reach your eye.
Step 4: Insert the Mirrors
Carefully slide your first mirror into the bottom slot. The reflective side should face upward at a 45-degree angle. Secure it with tape along the edges, make sure it's stable and won't shift around.
Insert the second mirror into the top slot with the reflective side facing downward at 45 degrees (opposite to the first mirror). Tape it securely.
Step 5: Join the Two Sections
If you're using two milk cartons, align them end-to-end and tape them together firmly with duct tape. Make sure the connection is light-tight, you don't want light leaking in through gaps, or your view will be washed out.
If you're using a single long tube, you're already connected. Just make sure both mirrors are secure.
Step 6: Test and Adjust
Hold your periscope vertically with the top opening facing away from you and the bottom viewing hole at your eye level. Look through the bottom hole and aim the top opening at something tall (a ceiling, a high shelf, or hold it over a fence).
Can you see the reflection clearly? If the image looks dim or cropped, you may need to adjust the angle of your mirrors slightly or make sure your viewing holes align properly with the mirror positions.
How to Use Your DIY Periscope
Now for the fun part. Take your periscope outside and try these viewing experiments:
Over-the-Fence Viewing: Stand on one side of a tall fence and hold your periscope vertically so the top opening peeks over the fence line. You'll see what's on the other side without climbing.
Around-the-Corner Espionage: Crouch at a corner and extend your periscope horizontally around the edge. You can spy on what's happening down a hallway or around a building without revealing yourself.
Crowd Periscope: At a parade or crowded event (with adult permission), hold your periscope up high to see over taller people's heads.
Nature Observation: Use it to peek into high bird nests, over tall grass, or into other hard-to-reach spots without disturbing wildlife.
The Science Behind the Reflection
When light travels from an object to your eye through the periscope, it's making two perfect 90-degree turns. Here's the physics breakdown:
Law of Reflection: The angle of incidence (how light hits a surface) equals the angle of reflection (how it bounces off). This isn't just a theory, it's a physical law that works every single time. Your periscope proves it.
Why 45 Degrees Is Magic: When you want to turn light exactly 90 degrees, a mirror tilted at 45 degrees does the job perfectly. Think of it like a mathematical billiard ball bounce, the angle going in determines the angle coming out.
Path of Light: Light from the object you're viewing enters the top opening, travels in a straight line until it hits Mirror #1, bounces downward at 90 degrees, travels straight down the tube, hits Mirror #2, bounces outward at another 90 degrees, and lands in your eye. Two mirrors, two bounces, zero light loss (well, almost zero: mirrors aren't perfect).
Frequently Asked Questions
Can I use a single mirror instead of two?
Not if you want a traditional periscope. One mirror would just reflect the image sideways or upward, but you wouldn't be able to look through the device comfortably. Two mirrors create the necessary path that redirects light from above (or around) down to eye level.
Why is my periscope image reversed?
If your image looks flipped horizontally, don't panic: that's actually normal! Each mirror reverses the image once. Depending on how you've positioned your mirrors, you might see a left-right flip. Some periscopes are designed to correct this with additional mirrors, but for a basic DIY version, the flip is expected.
What if my mirrors aren't exactly 45 degrees?
Close is usually good enough for a basic periscope, but you'll notice the difference in clarity. If the angle is too steep or too shallow, light won't make it to your eye: it'll hit the inside walls of the tube instead. Use a protractor when marking your slots if you want precision.
Can I make a longer periscope?
Absolutely! The longer the tube, the higher you can see over obstacles. Just remember that the farther light has to travel, the dimmer your image might get. Also, longer periscopes are harder to hold steady.
Do I need glass mirrors, or will reflective paper work?
Real mirrors give the clearest, brightest image. Reflective paper or aluminum foil can work in a pinch, but they don't reflect light as efficiently, so your view will be dimmer and less sharp.
Why This Experiment Matters
Building a periscope isn't just about making a cool spy gadget (though that's a solid bonus). You're learning fundamental principles of optics that apply to everything from submarine navigation to fiber optic cables to the camera in your phone.
Understanding how light behaves when it bounces: predictably, mathematically, every single time: opens the door to understanding lasers, telescopes, microscopes, and even how your eyes work. The 45-degree angle you cut today? That's the same angle engineers use in real periscopes on naval submarines.
Plus, you've created something functional with your own hands using materials that were probably destined for the recycling bin. That's hands-on physics with a side of creative problem-solving.
Want more optical experiments? Check out the rest of our 100-experiment series at Tierney Family Farms where we're proving that science education doesn't need a fancy lab: just curiosity, cardboard, and a willingness to get hands-on.
Now grab those mirrors and start building. The world looks different when you can see around corners.