The double-slit experiment at home

The question

If light is made of particles, why does it make a stripy pattern when it passes through tiny gaps?

Learn just enough first

Watch a short video on wave–particle duality and the double-slit experiment
Read the double-slit chapter of Quantum: A Guide for the Perplexed

Kit

A low-power red laser pointer (Class 2, under 1 mW, wavelength usually 650 nm — check the label)
A single hair, taped across a card with a hole cut in it
Kitchen foil and a craft knife, to scratch two very close slits
A wall or white card 2–3 m away, and a tape measure

My prediction

What pattern do I expect on the wall: one bright spot, two bright lines, or something else?

Method

Shine the laser past the hair onto the far wall, in a darkened room. Look for a row of bright and dark spots.
Measure the distance from the hair to the wall, $L$, and the spacing between neighbouring dark spots, $\Delta y$.
Now scratch two parallel slits in foil, as close as you can (under 0.5 mm apart), and repeat.
Photograph each pattern with a ruler held next to it.

Safety: Never look into the laser or point it at anyone. Watch out for reflections off shiny surfaces.

Data

Setup	Distance to wall $L$ (m)	Fringe spacing $\Delta y$ (m)
Hair
Foil double slit

Result

The fringe spacing links the wavelength $\lambda$, the distance $L$ and the gap or hair width $d$:

$$ \Delta y = \frac{\lambda L}{d} \quad\Rightarrow\quad d = \frac{\lambda L}{\Delta y} $$

Use it to work out the thickness of my hair (human hair is usually 50–100 micrometres) and the gap between my two slits.

Explain it to a friend

Light going through two gaps spreads out and overlaps with itself. Where the waves line up you get bright stripes; where they cancel, darkness. The strange quantum part: even when light is sent one photon at a time, the stripes still build up — each photon seems to go through both slits.

New questions

What happens to the pattern if I move the wall further away?
Why does a single hair make a pattern at all?