DIY Double-Slit Experiment at Home: Young's Setup with Laser Pointers (H2 Physics 2026)

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

Stick to visible Class 2 pointers (<1 mW, labelled to IEC 60825-1) for classroom use; keep the beam below eye level and avoid reflective surfaces.

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

A photograph plus ImageJ works too; include a ruler for scale.

7 Calculations

For small angles $\sin\theta \approx \tan\theta \approx x\space/\space D$ (see OpenStax College Physics 2e §27.3):

$$\lambda\space=\space\frac{d\space x}{D}$$

(Worked numerical example unchanged from earlier draft.)

8 Uncertainty Analysis

Combined, $\delta\lambda/\lambda \approx 7\space\%$ - for a 650 nm red laser that means $\lambda = 650 \pm 45$ nm.
You can reduce this by: (1) measuring 10 fringes (cuts $\delta x$ to ~0.5 mm / 10); (2) photographing the pattern for digital fitting; (3) measuring $d$ under a calibrated USB microscope instead of calipers.

9 Advanced Investigations (All Start With a Two-Slit)

1. Compare wavelengths: swap red, green, violet lasers; plot $x$ against $\lambda$.
2. Envelope exploration: slide a narrow card in front of one slit to vary its width $a$ while keeping $d$ fixed; watch missing orders reappear.
3. White-light two-slit: use a white LED plus pinhole and observe coloured fringes near the centre.
4. Variable $d$: mount one blade on a micrometer screw; plot $x$ against $1/d$ to verify linearity.

10 Common Problems and Fixes

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 Class 2 (<1 mW), beam perpendicular
• Room darkened, screen ≥2 m away
• ≥5 fringes measured for each run
• Uncertainties propagated (target $\leq$ ±5 % if you measure ≥10 fringes)
• Results compared with manufacturer's $\lambda$ label (expect agreement within a few percent)

Master this and you will have reproduced one of the most pivotal experiments in science - with drawer parts and a high-school ruler.

References

• Babinet's Principle - Wikipedia
• Double-slit Experiment - HyperPhysics
• Young's Interference Experiment - Physics LibreTexts
• Young's Double-Slit Experiment - OpenStax College Physics 2e §27.3
• IEC 60825-1 laser classification (Class 2 visible power limit <1 mW)

Want the matching write-ups for SHM, circular motion, and Paper 3 practical drills? Keep revising with our H2 Physics notes hub - every topic links back to experiment checklists like this two-slit build.

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