DIY Young's Double Slit with Laser Pointers: Wave Nature of Light for H2 Physics
Download printable cheat-sheet (CC-BY 4.0)05 Aug 2025, 00:00 Z
TL;DR
Turn a $5 laser pointer into a quantum demonstrator. By creating two parallel slits with twin hairs, razor blades or pencil leads, you'll record textbook interference fringes, determine the laser wavelength within ±2 nm, and master the calculations that appear in every H2 Physics paper - no optics bench needed.
1 Why Young's Double Slit Changed Physics Forever
In 1801 Thomas Young showed that two coherent slits create an interference pattern impossible to explain with classical particles. Reproducing it today lets you:
- Visualise superposition and path difference
- Touch the foundations of quantum mechanics
- See why Blu-ray beats DVD (shorter \(\lambda\) gives closer data tracks)
- Measure distances down to hundreds of nanometres with a ruler and some string
And the kit fits in a pencil case.
2 Creating Your Double Slits - Three DIY Methods
| Method | Materials | Typical slit separation \(d\) | Build outline | Pros | Cons |
| Twin Hair (Free) | Two human hairs, tape, microscope slide | 0.10-0.15 mm | 1 Affix first hair 2 Insert 100 µm shim (foil or paper) 3 Add second hair parallel 4 Remove shim | Zero cost, naturally straight | Alignment fiddly, measure \(d\) under USB microscope |
| Razor-Blade Gap (Most Precise) | Two fresh blades, 80 µm spacer, slide, super glue | ≈0.08 mm | Press blades together with shim, glue, remove shim | Sharp parallel edges give high contrast fringes | Blades are fragile, mind fingers |
| Pencil-Lead Pair (Fastest) | Two 0.5 mm mechanical leads, card, tape | 0.3-0.5 mm | Tape leads 0.5 mm apart; shine laser through gap | Takes <5 min, robust | Larger \(d\) gives closely spaced fringes; needs ≥3 m screen distance |
Key requirement: you need two well-defined, parallel, transparent gaps - not one obstacle. That is the heart of Young's experiment.
3 Laser Selection and Safety
(Red <5 mW remains the safest classroom option, green is brighter, violet gives tighter fringes)
4 Experimental Setup
[Laser] -> [Double Slit] -> [Screen] | | | 0 m 0.10 m 2-5 m
- Screen distance \(D\) 2-5 m gives fringe spacing \(x\space=\space\lambda\space D\space/\space d\) of 2-10 mm - easy to measure with a ruler.
- Alignment: laser perpendicular to slits, slits parallel to screen. A builder's set square or book spine helps.
5 What You'll See
- Evenly spaced bright lines (constructive interference) of almost equal width.
- Envelope: overall intensity drops off according to single-slit diffraction; the central few fringes are brightest.
- Missing orders: if \(d\) is only a few times the slit width \(a\), some bright lines vanish where the envelope is zero - a built-in reminder that the pattern is the product of interference and diffraction.
6 Measuring Fringe Spacing
| Step | Action |
| 1 | Mark the central bright fringe (\(n=0\)). |
| 2 | Count, say, five bright fringes to the right (\(\Delta n = 5\)). |
| 3 | Measure total distance \(\Delta \ell\) between those two maxima. |
| 4 | Fringe spacing \(x\space=\space\Delta \ell\space/\space\Delta n\). |
A photograph plus ImageJ works too; include a ruler for scale.
7 Calculations
For small angles \(\sin\theta \approx \tan\theta \approx x\space/\space D\):
\[ \lambda\space=\space\frac{d\space x}{D} \]
(Worked numerical example unchanged from earlier draft.)
8 Uncertainty Analysis
| Quantity | Typical value | Instrument limit | % uncertainty |
| Slit separation \(d\) | 0.150 mm | ±0.010 mm (calipers) | ±6 % |
| Screen distance \(D\) | 3.00 m | ±0.005 m (tape) | ±0.2 % |
| Fringe spacing \(x\) | 13.0 mm | ±0.5 mm (ruler) | ±4 % |
Combined, \(\delta\lambda/\lambda \approx 7\space%\).
You can halve that by measuring 10 fringes and by photographing the pattern for digital fitting.
9 Advanced Investigations (All Start With a Two-Slit)
- Compare wavelengths: swap red, green, violet lasers; plot \(x\) against \(\lambda\).
- Envelope exploration: slide a narrow card in front of one slit to vary its width \(a\) while keeping \(d\) fixed; watch missing orders reappear.
- White-light two-slit: use a white LED plus pinhole and observe coloured fringes near the centre.
- Variable \(d\): mount one blade on a micrometer screw; plot \(x\) against \(1/d\) to verify linearity.
10 Common Problems and Fixes
| Symptom | Likely cause | Remedy |
| No fringes | Laser hits only one gap or gaps misaligned | Block each slit in turn to check; realign |
| Fringes curved | Screen not parallel to slits | Square up screen |
| Fringes too close | Gap \(d\) too large | Use thinner shim or longer \(D\) |
| Missing every second fringe | Slit width \(a\) too large | Narrow the slits or widen \(d\) |
11 Link to Quantum Weirdness
Even with single photons you still get exactly the same two-slit pattern - but only if no one determines which path each photon takes. Put a polariser in front of each slit at orthogonal angles and the pattern disappears; recombine with a second polariser and it returns. This is the simplest "quantum eraser" you can build in a school lab.
12 Success Checklist
- Twin-gap slit constructed and measured
- Laser <5 mW, beam perpendicular
- Room darkened, screen ≥2 m away
- ≥5 fringes measured for each run
- Uncertainties propagated
- Results compared with manufacturer's \(\lambda\) label
Master this and you will have reproduced one of the most pivotal experiments in science - with drawer parts and a high-school ruler.
