Pathways Forward · Companion to Computing With Light
A hands-on unit of seven light experiments for a homeschool family, from a laser and a comb to a small optical calculator that does the same math as a photonic AI chip. It covers what's in the kit, what each experiment teaches, where to buy the parts, and what it costs.
The unit has seven short experiments and a capstone project. Each experiment is built around one idea that photonic chips depend on, so the student isn't just watching pretty rainbows. By the end, they have measured the wavelength of laser light with a tape measure, sent a signal down an optical fiber, and built a device that multiplies numbers using light.
The core kit costs about $235 in parts, not counting shipping. That price assumes the family already has a computer, a smartphone, and common household items. An optional add-on brings it to about $360: a real Michelson interferometer kit ($125). Less than half the core money goes to optics. The rest is a microcontroller and light sensors, which the family can reuse for other projects.
Physical sizes in this unit are in inches and feet. Light wavelengths are in nanometers (nm), because no customary unit exists at that scale; red laser light is about 650 nm, or roughly 1/40,000 of an inch. Diffraction gratings are sold by "lines per millimeter" (500 lines/mm is about 12,700 lines per inch). The formulas only use ratios of lengths, so the inch measurements work directly.
Buy Class 2 lasers (under 1 milliwatt). The blink reflex protects the eye from an accidental brief exposure at this power. Common "Class 3R" pointers (up to 5 mW) and cheap online pointers that don't list a class can cause eye injury. Many cheap green pointers also leak invisible infrared light.
Never point a laser at anyone's eyes, pets, vehicles, or aircraft, and never look into the beam, even a Class 2 beam, on purpose.
Watch for stray reflections. Mirrors, CDs, and gratings throw beams in unexpected directions. Work with the beam below seated eye level, and take off watches and rings.
At Class 2, no safety goggles are needed. If you ever move up to a stronger laser, buy goggles rated for that laser's exact wavelength.
Before you start · What light is
Light is a wave, like a ripple on a pond, except that it's a ripple in electric and magnetic fields and it moves at about 186,000 miles per second. The distance from one wave crest to the next is the wavelength. Our eyes see different wavelengths as different colors: the longest visible waves look red, and the shortest look violet.
These waves are extremely small. Red laser light has a wavelength of about 650 nm, which is 1/40,000 of an inch, so about 150 red waves fit across the thickness of one sheet of printer paper. You can't measure a wavelength directly with a ruler. Every experiment below uses a trick that turns something that tiny into a distance big enough to measure.
No trigonometry is needed anywhere in this unit. The math is multiplying, dividing, reading graphs, and one idea from geometry: similar triangles, meaning two triangles with the same shape but different sizes. Their sides are in the same proportions.
Photonics link: the Sydney chip computes by shaping how light diffracts through nanostructures.
Shine the laser through a diffraction grating at a wall 3 feet away. You'll see a row of bright dots. Measure two distances: y, from the center dot to the first dot beside it, and h, from the grating straight to that first dot (stretch the tape measure along the slanted line). Dividing one by the other gives the wavelength, as explained below. With a 500 lines/mm grating at 3 feet, a red laser's first dots land about 12½ inches from center and a green laser's about 10 inches. The student should get within a few percent of the labeled 650 nm and 532 nm.
Then use the same idea in reverse: shine the laser past a single human hair taped across a hole in an index card, with the wall 10 feet away. From the spacing of the dark gaps in the pattern, work out the hair's thickness, typically 0.002–0.004 inch. Check the answer against a micrometer if you have one.
The science
What diffraction is. Diffraction is the way a wave spreads out after it squeezes through a narrow opening or passes the edge of an object. Water waves passing through a gap in a breakwater, and sound reaching you around a corner, are both diffraction. Light does it too, but only noticeably when the opening is extremely narrow, close to the size of one wavelength. That's why you never see it through a doorway, but you do see it through the microscopic slits of a grating or around a single hair.
What a grating is. A diffraction grating is a clear sheet ruled with thousands of evenly spaced lines: 500 per millimeter, or about 12,700 per inch. The narrow clear gaps between the lines act as slits, and light spreads out from each one like ripples going through a gap in a breakwater.
What interference is. Interference is what happens when two or more waves overlap: they add together. If they meet crest-to-crest ("in step"), they build a bigger wave, which for light means brighter. If one wave's crests land on the other's troughs ("out of step"), they cancel, which for light means dark. Most places land somewhere in between. Two pebbles dropped in a pond show it: where the ripple rings cross, some spots bob extra high and some stay almost still.
Why you see dots instead of a smear. The ripples from all those slits overlap and interfere. In most directions, crests from one slit meet troughs from another and cancel out, leaving darkness. In a few special directions every slit's crests line up, and the light adds up to a bright dot. The straight-ahead direction always works. The first dot to the side appears in the direction where light from each slit travels exactly one wavelength farther than light from the slit next to it, so the crests still line up, just one wave apart.
Where similar triangles come in. In the close-up there is a tiny right triangle: its long side is the slit spacing d, and its short side is one wavelength λ. It has exactly the same shape as the big triangle across the room, whose long side is h and short side is y, because both point in the same direction. Triangles with the same shape have their sides in the same proportions:
For a 500 lines/mm grating, d = 1/500 mm = 2,000 nm. Worked example with the red laser: y = 12.4 in and h = 38.1 in, so λ = 2,000 × 12.4 ÷ 38.1 = 651 nm. The inches cancel out, so you never convert units.
Why red spreads farther than green. Red waves are longer, so the extra-distance segment has to be longer, and that means a steeper angle. Red's first dot lands about 12½ inches out, and green's about 10. That's why a grating spreads white light into a rainbow, which is the basis of Experiment 2.
The hair. A hair blocks a thin strip of light, and the pattern it makes is almost identical to the one a slit of the same width would make. The rule for a single slit is that narrower objects spread light wider: halve the thickness and the dark gaps move twice as far apart. For a red laser with the wall 10 feet away, the numbers work out to a simple formula:
Measure across several gaps and divide by the number of gaps to get an accurate spacing. Gaps about 1 inch apart mean a hair about 0.003 inch thick. (With the green laser, use 0.0025 in place of 0.0031.)
Going further, after trigonometry: this is the grating equation d·sin θ = m·λ. The ratio y ÷ h is sin θ.
Learns
Parts
Red and green Class 2 lasers, diffraction gratings, tape measure, a hair, an index card.
Photonics link: wavelength-division multiplexing, where many colors carry separate data streams in one waveguide.
Use a handheld spectroscope with a built-in nanometer scale to look at a series of light sources: sunlight reflected off a white card, an incandescent bulb, a white LED, a compact fluorescent bulb, and a streetlight. Sketch and label each spectrum. Next, build the free Public Lab papercraft spectrometer, which uses a piece of DVD as the grating. Mount it on a smartphone camera and compare its photos with what the handheld scope shows.
The science
How a spectroscope works. It's Experiment 1 inside a tube. A narrow slit lets in a thin line of light, and a grating spreads it out by wavelength. Longer (redder) waves bend farther, so each color lands in its own place, and a printed scale shows the wavelength of each position. A DVD works as a grating because its data tracks are a tightly spaced spiral, about 0.74 micrometers (millionths of a meter) apart.
Two kinds of light source give two kinds of spectrum.
Why a gas gives off exact colors. Each electron in an atom can only sit at certain energy levels, like the rungs of a ladder with nothing in between. When an electron drops from a higher rung to a lower one, it gives off a single packet of light. The size of the drop sets the color: a big drop makes bluer light, and a small drop makes redder light. Every element has its own ladder, and so its own pattern of lines, which works like a fingerprint. The bright blue and green lines in a fluorescent tube come from mercury. This is how astronomers can tell what distant stars are made of.
The white LED. A "white" LED is really a blue LED coated with a yellow-glowing phosphor. You'll see a sharp blue spike plus a broad yellow hump, and your eye blends the two into white.
Learns
Parts
Quantitative spectroscope, a printed papercraft template, an old DVD, a smartphone, household bulbs.
Photonics link: focusing light in and out of chips, which is a large part of the engineering in co-packaged optics.
Use a ray box that makes three colored beams, plus acrylic lenses and mirrors. Trace the rays on paper, find the focal points of convex and concave lenses, and measure how much a beam bends when it enters a flat acrylic face. From that, calculate acrylic's refractive index using the circle method below. If your kit has no flat-sided piece, use a clear, flat-sided plastic container filled with water. Then aim a beam at the inside of a flat face at a glancing angle to get total internal reflection, the effect that keeps light trapped inside an optical fiber.
The science
Why light bends. Light travels more slowly in acrylic than in air, at about two-thirds the speed. Imagine a marching band walking at an angle from pavement onto mud. The marchers who reach the mud first slow down first, and the whole line pivots. A light beam that enters a slower material at an angle pivots the same way, bending toward the normal, the imaginary line straight out from the surface. The refractive index (n) measures how much slower light goes: n = 1.49 for acrylic means light is 1.49 times slower inside it than in air.
Try several entry angles. The ratio a ÷ b stays about the same (near 1.5 for acrylic and 1.33 for water). A number that stays constant while the angle changes is the sign that you've found a real physical property of the material.
Lenses. A lens is refraction put to work. A convex lens (thicker in the middle) bends every parallel beam toward one point, the focal point. The distance from the lens to that point is the focal length. A concave lens (thinner in the middle) spreads beams apart. Trace the spreading beams backward with a ruler and they seem to come from a point in front of the lens.
Total internal reflection. Now reverse the direction: send light from inside the acrylic toward air. It bends away from the normal. Tilt the beam more and more, and eventually it bends so far that it can't get out at all, and 100% of it reflects back inside, better than any mirror. For acrylic this happens once the beam is more than about 42° from the normal (measure with the kit's protractor). Optical fibers use this effect, as Experiment 5 shows.
Learns
Parts
Tech Light Lab LED optics kit (ray box, lenses, mirrors, grating, protractor), paper, pencil, compass (for drawing circles), ruler.
Photonics link: optical modulators encode data by controlling light's phase and polarization.
Look through two polarizing filters and rotate one. Record how bright the light is at each 15° step, using the light sensor from Experiment 6 if it's already built, and compare with the prediction table below. Put a clear plastic fork or a CD case between two crossed polarizers and you'll see stress patterns as colored bands. Point a filter at a laptop or phone screen to show that LCD screens give off polarized light. Try the same with polarized sunglasses on a glare-covered surface.
The science
What polarization is. Polarization is the direction a light wave vibrates, and light is polarized when all of its waves vibrate the same way. Picture shaking a rope tied to a post: you can make it wiggle up and down, or side to side. Now pass the rope through the gap in a picket fence. Up-and-down wiggles slide through the slot, but side-to-side wiggles get blocked. A polarizing filter works like that fence for light. You meet polarized light every day: glare bouncing off water or a road is mostly polarized side to side, which is why polarized sunglasses (which block side-to-side light) cut it, and phone and laptop screens send out polarized light on purpose.
Light wiggles in a direction. A light wave vibrates side to side as it travels, and that side-to-side direction can point any way around the beam: up and down, left and right, or anything in between. Ordinary light from the sun or a bulb is a random jumble of every direction. A polarizing filter contains long, lined-up molecules that let through only the part of the wiggle that points one particular way. After the filter the light is polarized: every wave wiggles in the same direction.
In between. At angles between 0° and 90°, some of the light gets through, but the brightness doesn't fall off in a straight line. It stays bright at first, then drops quickly, then levels off near dark. Use this table as the prediction and compare your sensor readings to it:
| Angle between filters | 0° | 15° | 30° | 45° | 60° | 75° | 90° |
|---|---|---|---|---|---|---|---|
| Predicted brightness | 100% | 93% | 75% | 50% | 25% | 7% | 0% |
Stress colors. Stretched or squeezed plastic twists polarized light, and it twists each color by a different amount. Between crossed filters, the places where the plastic is under stress light up in rainbow bands. Engineers use this to find stress points in plastic parts.
Going further, after trigonometry: the table is Malus's law, brightness = cos²θ.
Learns
Parts
Two or more pieces of linear polarizing film, clear plastic items, a phone or laptop screen.
Photonics link: optical interconnect, the part of the field that is shipping in data centers in 2026.
Build the Industrial Fiber Optics IF-E22 kit, an LED transmitter and a photodetector receiver connected by plastic optical fiber. It needs a little soldering. Send an on/off signal down the fiber. Then bend the fiber tighter and tighter until the signal drops out, which shows where total internal reflection fails. Also examine the fiber sample pack, which includes glass fiber, plastic fiber, and a coherent bundle that carries an actual image.
The science
A pipe for light. An optical fiber has a clear core inside a cladding layer made of a material with a slightly lower refractive index. Light traveling down the core keeps hitting the core's wall at a glancing angle. That angle is past the total-internal-reflection limit from Experiment 3, so every bit of it bounces back in. The light zigzags down the fiber, even around gentle curves, and hardly any escapes. Glass fibers carry signals for miles this way.
Why tight bends leak. When the fiber curves sharply, light moving in a straight line hits the outer wall head-on instead of at a glance. Past the limit angle, it passes straight through the wall and escapes. That's why the signal fades as you tighten the bend, and why data-center cables have a "minimum bend radius" printed on them.
Light as data. The transmitter switches an LED on and off very fast: on means 1 and off means 0. At the far end, a photodetector turns each flash back into an electrical pulse. Real data-center links do the same thing billions of times per second, often on many colors at once (the idea from Experiment 2).
Learns
Parts
IF-E22 kit, IF-FGB2 fiber assortment, a soldering iron (if you don't have one), AA/9V battery per the kit.
Photonics link: every optical computer ends at a photodetector that turns light into a number an electronic chip can use.
Connect a digital light sensor to an Arduino (no soldering; the sensor plugs in with a cable). Log light readings to the computer. First measurement: move a small light source, such as a phone's flashlight LED, away from the sensor 6 inches at a time and plot brightness against distance. It should follow the inverse-square law. Reuse the sensor to take the readings in Experiment 4 and to build the capstone.
The science
How the sensor "sees." Inside the sensor is a tiny patch of silicon. Each packet of light that hits it can knock an electron loose, creating a small electric current. More light means more current. A chip in the sensor measures that current and reports it to the Arduino as a number in lux, a standard unit of brightness. Solar panels and phone cameras work the same way.
The inverse-square law. Light from a small source spreads out in every direction. At twice the distance, the same light covers a patch twice as wide and twice as tall, so four times the area, and each spot gets only a quarter as much. At three times the distance, it covers nine times the area and each spot gets a ninth.
How to check it. Multiply each reading by the distance, then by the distance again. If the law holds, you get about the same answer every time. (A flashlight with a shiny reflector focuses its beam and won't follow the law closely. That's worth trying too, as a lesson in why a result can disagree with a prediction.)
Learns
Parts
Arduino UNO R4 WiFi (it has the plug-in sensor port built in), two BH1750 light sensors, cables, a phone flashlight or small LED bulb.
Photonics link: photonic AI accelerators are built from meshes of interferometers (Mach-Zehnder type), each one a tunable "weight."
A beam splitter divides the laser beam into two paths that bounce off mirrors and recombine. Where they meet, you see a pattern of light and dark rings. Press gently on the table, or hold a warm hand near one arm, and the rings shift. That's a length change smaller than a millionth of an inch showing up by eye. Count the rings as you turn the mirror screw to measure a very small movement.
The science
Waves can add or cancel. When two light waves meet crest-to-crest ("in step"), they add up to a brighter wave. When one wave's crests meet the other's troughs ("out of step"), they cancel and you get darkness. This is interference, the same effect that made the dots in Experiment 1 (defined there too, if you skipped ahead).
How the interferometer uses it. The beam splitter is a half-silvered mirror. Half the light goes through it to one mirror, and half reflects to another. Both beams bounce back and recombine on the screen. Whether they meet in step or out of step depends on whether the two round trips are the same length or differ by half a wave.
Why it's so sensitive. Moving a mirror by half a wavelength lengthens that beam's round trip by a whole wavelength, so the pattern changes from bright to dark and back to bright. One ring passes by. For a red laser, half a wavelength is about 1/80,000 of an inch. If you count 100 rings going by, the mirror has moved about 0.0013 inch, a distance you measured with no ruler at all. Warmth from a hand makes the metal expand slightly, and that is enough to shift the rings.
Why photonic AI cares. Put a small, adjustable delay in one arm and you control how much light comes out of each exit. That's a dial that sets a number, which is exactly what a "weight" is in a neural network. Photonic chips pack hundreds of tiny interferometers onto one chip.
Learns
Parts
IFO Michelson interferometer kit (includes steel table, mounts, beam splitter, mirrors, lens); uses the red laser from Experiment 1.
The main operation in AI is the dot product: multiply each input by a weight, then add up the results. A photonic chip does this with light. The input values set how bright each light source is, the weights set how much light each path lets through, and one detector adds everything up automatically because it measures the total light that lands on it. This capstone builds a slow, cardboard version of the same idea.
| Pattern shown (1 = LED on) | Sheet A (top) weights 0.9 · 0.9 · 0.1 · 0.1 | Sheet B (left) weights 0.9 · 0.1 · 0.9 · 0.1 | Answer |
|---|---|---|---|
| Top line: 1 · 1 · 0 · 0 | 0.9 + 0.9 = 1.8 | 0.9 + 0.1 = 1.0 | "top" |
| Left line: 1 · 0 · 1 · 0 | 0.9 + 0.1 = 1.0 | 0.9 + 0.9 = 1.8 | "left" |
Negative weights. Gray squares can only pass light or block it; they can't subtract. That means every weight is positive. The student discovers the usual fix on their own: two channels, with one reading subtracted from the other. That's also how real photonic systems handle negative weights.
A problem one layer can't solve. Challenge the student to design one sheet that fires for any horizontal line (top or bottom) but not for any vertical line (left or right). Trying to do it with the add-up arithmetic shows it's impossible. Making the top line score higher than the left line forces top-right to outweigh bottom-left, while making the bottom line win forces the opposite. This is a famous result from the 1960s, and it's the reason real neural networks stack several layers. The student will also hit stray light, uneven LEDs, and sensor noise. These are the real engineering problems listed in the brief's "Unsolved problems" section, in miniature.
These prices were checked at the supplier sites on September 22, 2026. Items marked est. are typical prices where one specific listing couldn't be confirmed. Several educational suppliers show items as temporarily sold out, so check alternates before ordering.
| Item | Used in | Where to buy | Cost |
|---|---|---|---|
| "Super Safe" red laser pointer, 650 nm, Class 2 (<1 mW) | 1, 5, 7 | Laser Classroom · alt: Arbor Scientific | $24.95 |
| "Super Safe" green laser pointer, 532 nm, Class 2 (<1 mW) | 1 | Laser Classroom | $37.95 |
| Diffraction gratings, 500 lines/mm, 5-pack | 1 | Laser Classroom · alt: Rainbow Symphony (25 slides, $15) | $9.95 |
| Quantitative spectroscope (400–700 nm scale) | 2 | Arbor Scientific (restock expected Oct. 2026) | $10.00 |
| Papercraft spectrometer template | 2 | Public Lab (free download) + an old DVD | $0 |
| Tech Light Lab LED optics kit (RGB ray box, 3 lenses, 2 mirrors, grating, protractor) | 3 | Laser Classroom | $54.95 |
| Linear polarizing film, a few sheets | 4 | Amazon (search) · higher grade: Edmund Optics ($47.75) | $12 est. |
| IF-E22 fiber optic communication kit | 5 | Industrial Fiber Optics | $20.00 |
| IF-FGB2 deluxe fiber assortment (12+ samples) | 5 | Industrial Fiber Optics | $6.00 |
| Arduino UNO R4 WiFi | 4, 6, capstone | Arduino Store · alt: SparkFun | $27.50 |
| Adafruit BH1750 light sensor, STEMMA QT (×2) | 4, 6, capstone | Adafruit | $9.00 |
| STEMMA QT / Qwiic cables (×2) | 6, capstone | Adafruit | $2 est. |
| Capstone parts: 4 white LEDs + resistors, jumper wires, breadboard, transparency printing | capstone | Adafruit / SparkFun / local copy shop | $20 est. |
| Core kit total (before shipping) | ≈ $235 | ||
| Item | Why | Where to buy | Cost |
|---|---|---|---|
| Michelson interferometer kit 45-942 | Experiment 7. The best hands-on connection to how photonic accelerators work. | Industrial Fiber Optics | $125.00 |
| Speed of Light apparatus kit (IF-SLK) | Measures the speed of light; a strong science-fair project. | Industrial Fiber Optics | $135.00 |
| Optical Voice Link kit (IF-OVL10K) | Send audio, not just on/off, over fiber. | Industrial Fiber Optics | $49.35 |
| Soldering iron starter kit | Only needed if the family doesn't have one (Experiment 5). | Any hardware store / SparkFun | $25 est. |
Tape measure, protractor, ruler, index cards, tape, cardboard boxes and tubes, white paper, a flashlight, a comb, an old CD and DVD, a glass of water, a clear plastic fork or CD case, polarized sunglasses, a smartphone, and a computer to program the Arduino.
With only one laser (red only), the core kit drops to about $195. A co-op of several families can share the Arduino and interferometer and still give each student their own lasers, gratings, and spectroscope. Rainbow Symphony's 25-slide grating pack and Arbor Scientific's 20-pack of 2-inch polarizers (about $70) are good choices for a group.
This assumes about two sessions a week of 1 to 1½ hours each.
| Weeks | Work |
|---|---|
| 1 | Safety rules; Experiment 3 (ray box, lenses, refractive index) |
| 2–3 | Experiment 1 (grating wavelength measurement, hair thickness) |
| 4 | Experiment 2 (spectroscopy; build the papercraft spectrometer) |
| 5–6 | Experiment 6 (Arduino and sensor, inverse-square law) |
| 7 | Experiment 4 (polarization, Malus's law with sensor data) |
| 8–9 | Experiment 5 (solder and test the fiber link) · Experiment 7 if purchased |
| 10–12 | Capstone: build, calibrate, classify, write up results |
Every experiment ends with a one-page lab write-up: question, setup sketch, data table, graph, result with an error estimate, and one sentence connecting it to photonic computing. The write-ups add up to a lab notebook that works as a portfolio piece for a dual-enrollment application or a summer research program.