Young’s double-slit experiment

Read how far the wave from each slit has travelled to reach a point on the screen — with light, microwaves or sound.

S₁ and S₂ are the two slits · P is the marker on the screen

λ 600 nm

d 0.30 mm

D 1.50 m

not to scale

The screen · distances along it, as a ruler would measure them

The apparatus

Wavelength λ 600 nm

Slit separation d0.30 mm

Slits to screen D1.50 m

A laser, two slits on a slide, a screen across the room.

The marker P

distance along screen y

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S₁P

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S₂P

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path difference

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in wavelengths

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angle θ from the centre

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d sin θ

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Measuring the pattern

fringe spacing s

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across 10 fringes

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Measured off the screen above, the way a ruler would: across several fringes at once, then divided.

Zoom the screen to fit the pattern

Showing 44.0 mm of screen.

Graph

points recorded

gradient

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Coherence

Two independent sources

Normally both slits are lit by the same source, so the two waves leave in step and stay in step. Switch this on and each slit becomes a source of its own, with no fixed phase relationship — as two separate lamps, or two ungated speakers, would be.

Overlays

Scale along the screen Number the bright fringes The little triangle at the slits Mark the span the spacing is measured across

The triangle treats the two paths as parallel. Compare its short side, d sin θ, with the exact path difference above: on a bench this shape they agree to about one part in a million.