The 42nd Period | STEM Education Hub

Build a working periscope, magnifier, or simple telescope out of mirrors and lenses. Then prove it does what you said it would: predict its performance, measure it, and show it working in a short demo.

🎯 Goals

Choose a device and write a spec for it with numbers you can measure.
Measure your own lenses and mirrors, and predict your device’s performance by calculation.
Build and align the device, and measure what it actually does.
Compare prediction with measurement, calculate a percent error, and show the device working in a short demo.

🧰 Materials

🔍A set of converging lenses (for example f ≈ 5, 10, 20, 30, and 50 cm). Measure each one yourself

🪞Two small flat mirrors, or two 45° right-angle prisms

📦Cardboard tubes, boxes, or poster board, plus black paper

🧵Tape, modeling clay, rubber bands, and glue

📏A meter stick, a ruler with millimeter marks, and a protractor

🔦A laser pointer, for aligning and testing a periscope

📱A phone for recording your demo

🧮Calculator

📓Physics notebook

📏 Constraints

Build one device: a periscope, a magnifier, or a simple telescope.
Write your spec before you build. It needs at least two requirements with numbers you can measure.
Measure the focal length of every lens you use and the angle of every mirror. Don’t trust labels or assumptions.
Predict your device’s performance by calculation before you test it, and show your steps.
Test with a method that gives a number with units, not just “it looks right.”
Report the predicted and the measured values, a percent error, and an explanation of any gap.
Give a 60–90 second demo, live or recorded, that shows the device working and shows your measurement.
Safety: never look at the Sun through any optical device. Keep laser beams below eye level and never aim one at a face. Handle mirror edges carefully.

DAY 1

Choose & Plan

Choose your device.

Pick the one you want to build. The guide below changes to match, with spec ideas, design rules, build tips, and a test method. The rest of the page follows your choice.

🔭 Simple Telescope Guide

Spec ideas (use or adapt)

Angular magnification M of at least 4×.
A tube length of 60 cm or less.
Resolve the lines of a ruled sheet, or the mortar between bricks, at 8 m.

Design rules

M = f_objective ÷ f_eyepiece.
For a faraway object, the lenses sit one total focal length apart: length ≈ f_objective + f_eyepiece.
For a nearby object, move the eyepiece back slightly, since the objective’s image forms beyond its focal point.
The image is inverted, which is fine for this project.

Build tips

Choose a long-focal-length lens for the objective and a short one for the eyepiece, and measure both yourself.
Mount them in tubes that slide, such as a tube-in-tube design, so the spacing can change.
Aim at a distant window or a brick wall, and slide the eyepiece until the image is sharp.

How to test it

Look at a regular pattern, such as bricks or a ruled sheet 8–10 m away, with one eye through the telescope and the other eye unaided.
With both eyes open, count how many unaided-eye bricks fit across one brick in the telescope’s view. That count is M.
Measure the tube length from lens to lens at the sharp-focus position.

Write your spec.

Turn “a good device” into numbers. Write at least two requirements, with a target for each and how you’ll measure it. Use or adapt the spec ideas in your guide.

Requirement	Target value	How I will measure it

Measure your parts.

Never trust a label. Measure the focal length of each lens by focusing a distant window on a card (Activity 6-1-3), and measure the angles and sizes of the mirrors, tubes, and windows you plan to use.

Part	Measured value (with units)	How I measured it

Warm up with the design math.

These four problems are for the simple telescope. They are the calculations you’ll use to predict and judge your device.

Simple Telescope Warm-Up

A correct answer is within the tolerance shown by the checker.

An objective with f = 40 cm and an eyepiece with f = 10 cm. What is the magnification?M = f_objective ÷ f_eyepiece

How long is that telescope for a faraway object?length ≈ f_objective + f_eyepiece

With a 45 cm objective, what eyepiece focal length gives M = 5?

You predicted M = 4.0 and measured M = 3.6. What is the percent error?|measured − predicted| ÷ measured × 100

Predict your device’s performance.

Using your measured parts, calculate what your device should do on each requirement, before you build. Write each calculation in your notebook, and put the answers in the prediction column.

Quantity	Predicted (from my calculation)	Measured (Day 3)	% error (Day 3)

📓 Physics Notebook

Sketch your design with dimensions, and include a ray diagram (mirror angles for a periscope, or the two focal points for a lens device). Explain why the numbers you chose should meet your spec.

DAY 2

Build

Build to your design.

Follow your simple telescope build tips and your sketch. Keep every dimension in your notebook, and change your sketch whenever the build changes.

Align and focus.

Slide the eyepiece until a distant scene is sharp, then lock the tubes. Mark the spacing, and measure it. It should be near the sum of the two focal lengths.

What good progress looks like

By the end of today, your device should be assembled and give a recognizable picture, even if it isn’t sharp yet. Don’t fix problems by guessing. Measure, compare with your prediction, and change one thing at a time.

DAY 3

Test & Tune

Measure what your device really does.

Use the test method in your guide. Each method gives a number: a miss distance for the periscope, a magnification for the magnifier or the telescope. Enter your measurements in the checker.

Telescope Checker

Enter the focal lengths you measured, the magnification you measured with the two-eye method, and (optionally) the tube length at sharp focus.

Objective f (cm)Eyepiece f (cm)Measured M (×)Measured length (cm)Target M (×)

Sample data (use only if you can’t run the tests)

Objective f = 41.0 cm, eyepiece f = 9.8 cm, measured M = 3.8×, and a tube length of 51 cm. That’s a predicted M of 4.18 and about a 10% error.

Compare with your prediction and tune.

Fill in the Measured and % error columns of your prediction table. If your device misses a spec, work out why, change one thing, and test again. Record each trial.

Trial	Measured value (with units)	What I changed before this trial

📓 Physics Notebook

Where did your device differ from your prediction, and by how much? List at least two likely causes, such as a lens that wasn’t at its measured focal length, a mirror that moved, or the limits of reading a ruler, and say which one you think mattered most.

DAY 4

Demo & Document

Plan and record your demo.

The demo is 60–90 seconds, live or recorded. Say what the device is and state your spec. Show it doing its job, and show your measurement on camera, such as the laser dot on the wall, the two rulers overlaid, or the brick count. Then state your predicted value, your measured value, and your percent error.

Write your report.

Write a claim, evidence, and reasoning paragraph. Claim: does your device meet its spec? Evidence: your measured numbers next to your spec and your prediction. Reasoning: which optics ideas from the unit explain the result, and why the gap is what it is.

Question 1

State your spec and show with numbers whether your device met it.

Question 2

Compare your predicted and measured performance. What was the percent error, and what are the likely causes of the gap?

Question 3

Which idea from the unit (the law of reflection, refraction and total internal reflection, or the lens equation) mattered most to your design, and how did you use it?

Question 4

What single change would most improve your device? Support it with a calculation.

📤 Turn In

Your working simple telescope, ready to demonstrate
Your spec sheet, your parts measurements, and your prediction calculations
Your measurements, your prediction table with percent errors, and your trial log
Your 60–90 second demo (live or recorded)
Your claim–evidence–reasoning report, and written answers to the four Conclusion questions

← Back to Project 6-1-4 Overview Review the Deep Dive →Next up: Problem 6-2-1, Vision Aid Design.