The Double Slit — Measuring the Wavelength of Light

Shine a laser through two fine slits, watch light split into bright and dark bands, and measure its wavelength with a ruler

Beginner🕐30 min●Low hazardoptics waves interference measurement

Summary

Shine a laser through a pair of slits scratched a fraction of a millimetre apart and the single beam fans out into a row of evenly spaced bright and dark bands on the wall. Those bands are interference ¹ fringes — direct evidence that light travels as a wave. Better still, the spacing of the fringes is tied to the wavelength ² of the light by a simple formula, so with a ruler and a little arithmetic you can measure the wavelength of a laser — a length of a few ten-thousandths of a millimetre — to within a few percent.

History

For over a century after Newton, most physicists believed light was a stream of tiny particles. In 1801 Thomas Young ³, an English physician with a gift for languages and a habit of solving whatever puzzle crossed his desk, did something disarmingly simple: he let sunlight pass through a pinhole, then through two closely spaced slits, and caught the result on a screen. He saw not two bright lines but a series of coloured bands. Particles could not do that; only waves, adding crest-to-crest in some places and crest-to-trough in others, could. It was the first convincing demonstration of the wave nature of light.

Young’s result was so at odds with Newtonian orthodoxy that it was ridiculed for years. Vindication came from Augustin-Jean Fresnel ⁴, who in the 1810s built the wave theory into rigorous mathematics, and finally from James Clerk Maxwell ⁵, whose 1865 equations revealed light to be an electromagnetic wave. The double slit has never lost its power to surprise: when the experiment is repeated one photon ⁶ at a time, each particle of light lands as a single dot — yet the interference pattern still builds up, dot by dot, as though each photon passes through both slits at once. Richard Feynman called it the experiment that holds “the heart of quantum mechanics.”

Hazards & preparation

Warning

Safety: A laser pointer is a Class 2 or Class 3R device. The beam is bright enough to damage the retina.

Never look into the beam or its mirror-like reflections, and never aim it at anyone’s eyes. Beware glints off watch faces, glossy tabletops, and glass.
Set the beam path at chest or table height, not eye height, so a stray reflection can’t travel straight into someone’s eyes.
Children should aim and switch the laser only under supervision. A green laser looks far brighter than a red one of the same power — treat it with extra respect.

Materials

A laser pointer — a cheap red pointer (about 650 nm) is perfect; green (about 532 nm) gives brighter fringes. Note the wavelength printed on the label so you can check your answer at the end.
A double slit — the crisp option is a manufactured slit slide, but you can make your own: coat a microscope slide or a piece of glass with soot from a candle, or paint it with opaque marker, then draw two parallel lines very close together with a sharp blade and a straightedge, scratching through to the glass.
A single human hair or a fine wire — an excellent stand-in that produces the same fringe pattern by diffraction ⁷ around it.
A screen — a white wall, a sheet of paper, or a card, 1–3 m away.
A tape measure or metre stick, and a ruler marked in millimetres.
A small clamp or blob of putty to hold the laser rock-steady — you cannot hold it by hand and measure fringes.
A dark room. The dimmer the room, the clearer the fringes.

Procedure

Part 1: See the fringes

Clamp the laser so it points at the screen and stays put. Switch it on. You should see a single bright dot.
Place the double slit a few centimetres in front of the laser so the beam covers both slits at once. Nudge it until the dot on the wall blossoms into a row of dots — a bright central spot flanked by weaker spots on either side, evenly spaced.
Darken the room. The pattern sharpens into a line of bright fringes separated by dark gaps. If you used a slit slide you may also see the whole row sitting inside a broader, gently fading envelope — that larger pattern is single-slit diffraction; the fine fringes inside it are the two-slit interference you want.
No slit handy? Hold a single hair in the beam instead. It casts not a shadow but a row of fringes — the same physics, produced by diffraction around the hair.

Part 2: Measure the wavelength

Fix the distance \(L\) from the slits to the screen — somewhere between 1 and 3 m. The larger \(L\) is, the wider the fringe spacing and the more accurate your measurement. Measure \(L\) with the tape and write it down.
On the screen, the fringes are close together, so don’t measure one gap — measure across many. Mark the centre of a bright fringe near the left edge of the pattern and the centre of one near the right edge, count the number of gaps \(N\) between them, and measure the total distance \(\Delta y_{\text{total}}\) with the millimetre ruler. The fringe spacing is \(\Delta y = \Delta y_{\text{total}} / N\).
You need the slit separation \(d\). Use the value printed on a manufactured slide, or measure it under a microscope. For a hair, the “slit width” is the hair’s thickness — typically 0.05–0.10 mm.
Put the numbers into the fringe formula (derived below) and solve for the wavelength: \[\lambda = \frac{d\,\Delta y}{L}\]
Compare with the wavelength printed on the laser. Landing within about 10% with a home-made slit is a good result.

Part 3: Measure the thickness of a hair

By turning the math around, you can use a laser of known wavelength to measure something incredibly small—like the exact thickness of your own hair.

Tape a single strand of hair tightly across a hole punched in a piece of card.
Clamp the card so the hair sits in the centre of the laser beam, throwing a diffraction pattern onto the screen.
Read the exact wavelength \(\lambda\) printed on your laser pointer (e.g., 650 nm for a typical red laser, or \(6.5 \times 10^{-7}\) m).
Measure the distance to the screen \(L\) and the fringe spacing \(\Delta y\), just as you did in Part 2.
Rearrange the formula to solve for the hair’s thickness, \(d\): \[d = \frac{\lambda L}{\Delta y}\]
Calculate your result. Most human hair falls between 0.05 mm (50 µm) and 0.10 mm (100 µm) — you have just measured it with a ruler and a laser!

What you should see

In a dark room, a clean row of sharp red (or green) dots marching out to either side of a bright central spot, all equally spaced, fading gradually toward the edges. Slide the screen farther away and the whole pattern stretches out proportionally; the fringe spacing grows but the number of fringes stays the same. Swap a red laser for a green one and the fringes crowd closer together — shorter wavelength, tighter spacing — a result you can see before you measure it.

Symptom	Likely cause	Fix
Only one dot, no fringes	Beam hitting just one slit, or slits too far apart	Move the slit so the beam straddles both lines; scratch the pair closer together
Fringes too faint to measure	Room too bright, or screen too close	Darken the room fully; move the screen farther back to spread the pattern
Broad blur, no sharp fringes	Slits too wide or ragged	Re-scratch finer, straighter lines; try a single hair instead
Uneven, lopsided spacing	Screen not square-on to the beam	Aim the laser perpendicular to the screen
Measured wavelength far off	Wrong slit separation \(d\), or too few gaps counted	Re-measure \(d\); measure across as many fringes as possible

The Science

Two slits, two paths

When the laser beam reaches the pair of slits, each slit acts — by Huygens’ principle ⁸ — as a fresh source of coherent ⁹ waves spreading outward. Because the two sources start from the same beam, they stay in lockstep. Beyond the slits, their waves overlap and add by superposition ¹⁰.

At any point on the screen, the wave from one slit has travelled a slightly different distance than the wave from the other. This path difference ¹¹ decides everything. Where it is a whole number of wavelengths, the two waves arrive crest-on-crest and reinforce — constructive interference ¹², a bright fringe. Where it is a half-wavelength out (an odd number of half-wavelengths), they arrive crest-on-trough and cancel — destructive interference ¹³, a dark gap.

The bright-fringe condition

For two slits a distance \(d\) apart, geometry gives the path difference to a point seen at angle \(\theta\) from straight ahead as \(d\sin\theta\). Bright fringes therefore appear wherever

\[d\sin\theta = m\lambda, \qquad m = 0, 1, 2, \ldots\]

Here \(m\) is the order of the fringe: \(m = 0\) is the bright central spot (equal path, zero difference), \(m = 1\) the first fringe out on each side, and so on. This single equation says that wider slit separation \(d\) packs the fringes closer together, and longer wavelength \(\lambda\) spreads them farther apart — exactly what you see when you switch from green to red.

From angle to a ruler measurement

The angles are tiny — a fraction of a degree — so we can use the small-angle approximation \(\sin\theta \approx \tan\theta = y / L\), where \(y\) is the distance of a fringe from the centre on a screen a distance \(L\) away. Substituting into the bright-fringe condition, the \(m\)-th fringe sits at \(y_m = m\lambda L / d\), so the spacing between neighbouring fringes is

\[\Delta y = \frac{\lambda L}{d}.\]

Rearranged, that is the working formula from the procedure, \(\lambda = d\,\Delta y / L\). Everything on the right is something you can measure with a ruler and a tape.

A worked example

Suppose your slits are \(d = 0.20\) mm apart, the screen is \(L = 2.0\) m away, and you measure 10 fringe gaps spanning 65 mm, so \(\Delta y = 6.5\) mm. Then

\[\lambda = \frac{d\,\Delta y}{L} = \frac{(0.20\times10^{-3}\,\text{m})(6.5\times10^{-3}\,\text{m})}{2.0\,\text{m}} = 6.5\times10^{-7}\,\text{m} = 650\ \text{nm},\]

a red wavelength — bang on for a typical red pointer. You have just measured a length smaller than a hundredth of the width of a human hair, using nothing but a ruler.

Questions to Explore

Red versus green. Before measuring, predict what happens to the fringe spacing when you swap a red laser for a green one at the same distance. Then try it.

Hint / answer

Green light has a shorter wavelength (about 532 nm) than red (about 650 nm). Since \(\Delta y = \lambda L / d\), the fringe spacing is proportional to wavelength, so the green fringes sit closer together — roughly 18% tighter. The pattern visibly “compresses” toward the centre.
Move the screen. What happens to the fringe spacing if you double the distance \(L\) from slits to screen? Does the wavelength you calculate change?

Hint / answer

The spacing \(\Delta y = \lambda L / d\) is proportional to \(L\), so doubling \(L\) doubles the spacing — the pattern spreads out. The calculated wavelength does not change: \(\lambda = d\,\Delta y / L\) has \(L\) in the denominator, and \(\Delta y\) grew in exact proportion, so the ratio is fixed. That is why a larger \(L\) gives a more accurate measurement without biasing it.
Cover one slit. Predict what happens to the fringes if you block one of the two slits completely.

Hint / answer

The sharp interference fringes vanish — there is no second wave to interfere with. You are left with the broad, gently fading pattern of a single slit diffracting on its own. This confirms the fringes come from the two paths interfering, not from anything within one slit.
How thick is a hair? If you use a hair instead of slits, you can turn the experiment around: knowing the laser’s wavelength, measure the hair’s thickness. How?

Hint / answer

A thin obstacle produces fringes governed by the same \(\Delta y = \lambda L / d\) relationship, with \(d\) now the hair’s width. Rearrange to \(d = \lambda L / \Delta y\), measure the fringe spacing, and solve for \(d\). Human hair comes out around 0.05–0.10 mm — a genuine micrometer measurement made with a ruler and a laser.
Add more slits. A diffraction grating ¹⁴ has not two but thousands of slits. How would you expect its pattern to differ from the double slit?

Hint / answer

More slits make each bright fringe far narrower and brighter, with wide dark gaps between the orders — the bright-fringe positions still obey \(d\sin\theta = m\lambda\), but they become razor-sharp. That sharpness is why gratings, not double slits, are used in real spectrometers to separate wavelengths precisely.

Going further

Build a simple spectrometer: shine white light (not a laser) through a fine grating or a CD’s rainbow surface and watch it fan into a spectrum — each wavelength bends to its own angle by \(d\sin\theta = m\lambda\).
Repeat the measurement with several lasers of different colours and plot measured wavelength against fringe spacing; the straight line through the origin is the formula made visible.
Read about the single-photon version of this experiment — the same apparatus, run so faintly that only one photon is in flight at a time, still builds the fringes and sits at the foundation of quantum mechanics.

Footnotes

Interference — The addition of two or more overlapping waves, producing regions that reinforce (bright) and cancel (dark).↩︎
Wavelength — The distance between successive crests of a wave; it sets the colour of visible light.↩︎
Thomas Young — English polymath (1773–1829) whose double-slit experiment (1801) demonstrated the wave nature of light.↩︎
Augustin-Jean Fresnel — French physicist (1788–1827) who put the wave theory of light on a rigorous mathematical footing, explaining diffraction.↩︎
James Clerk Maxwell — Scottish physicist (1831–1879) whose equations showed light to be an electromagnetic wave.↩︎
Photon — A single quantum of light — the smallest indivisible packet of electromagnetic energy.↩︎
Diffraction — The bending and spreading of a wave as it passes an edge or through a gap comparable in size to its wavelength.↩︎
Huygens’ principle — Every point on a wavefront acts as a source of secondary wavelets; their envelope forms the next wavefront.↩︎
Coherence — A fixed, steady phase relationship between waves, required for them to produce a stable interference pattern.↩︎
Superposition — The principle that overlapping waves add together point by point, their displacements summing algebraically.↩︎
Path difference — The extra distance one wave travels compared with another; a whole number of wavelengths gives a bright fringe.↩︎
Constructive interference — When two waves meet in step (crest on crest), adding to make a larger wave — a bright fringe.↩︎
Destructive interference — When two waves meet out of step (crest on trough), cancelling to make a smaller wave — a dark fringe.↩︎
Diffraction grating — A surface ruled with many closely spaced slits or lines that spreads light into sharp, widely separated orders.↩︎