Double-slit interference (Young)

The most famous interference experiment is the double slit, carried out by Thomas Young in 1801. Light from a single source passes through two narrow slits, a distance $d$ apart, and falls on a screen placed at a distance $D$. The two slits behave as two coherent sources: the waves leaving them interfere, and on the screen a regular succession of bright fringes (constructive interference) and dark fringes (destructive interference) appears.

The spacing between two consecutive fringes is:

Key formula — Fringe spacing

$\ev{\Delta y = \frac{\lambda\,D}{d}}$ where $\lambda$ is the wavelength, $D$ the distance to the screen and $d$ the distance between the slits.

The formula says intuitive things: the fringes are wider (easier to see) if the wavelength is large, if the screen is far away, or if the slits are very close together. By measuring $\Delta y$, $D$ and $d$ one can therefore derive the wavelength of the light — and this is how Young measured it for the first time.

Deep dive — Young's experiment

Young’s experiment was the first convincing demonstration of the wave nature of light, in contrast with Newton’s corpuscular theory. Today the same experiment can be repeated with electrons, atoms and even large molecules: they too produce interference fringes, demonstrating the wave–particle duality at the heart of quantum mechanics.

▶ The Double Slit Experiment: Light As A Wave — Up and Atom

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Topics: Onde Concepts: Interferenza

Related exercises: Interferenza di due onde identiche · Problema — Frange di Young · Sfida doppia fenditura ottica