In 1666 Isaac Newton let a narrow beam of sunlight through a small hole into a darkened room and passed it through a glass prism. Instead of a round white patch, it cast an elongated band of colour on the far wall — red bent least, violet bent most. Newton's rivals blamed the glass or the hole; his "experimentum crucis" proved the colours were already present in white light, each refracted by its own fixed amount, unified by nothing more than the glass's refractive index depending on wavelength.
n(color) = n0 + k·t, t = 0 (red) … 1 (violet)
sin(θ1)·n1 = sin(θ2)·n2 (Snell's law at every face)
- Incidence angle — tilts the prism against the fixed incoming beam, changing how sharply each colour bends; near the historical minimum-deviation position the spectrum is narrowest and sharpest.
- Prism apex angle — the glass wedge's own angle; a wider apex forces a longer path through glass and a wider spectrum.
- Slit aperture — Newton first used a wide hole and got a blurred oval; narrowing it (as here) isolates each colour's path and reveals a clean, elongated spectrum — the detail that led him to the right explanation.
- Dispersion strength — how much the glass's index changes across the visible range; real crown glass separates colours by only a degree or so, exaggerated here to make the geometry visible.
- Converging lens — Newton also showed the split was reversible: a lens (or a second, inverted prism) can bend the diverging colours back to a single point, recomposing white light and proving nothing was added or destroyed by the glass.
Real-world relevance: this single afternoon experiment ended a century of debate about the nature of colour and founded physical optics — the direct ancestor of the spectrometer and every colour-dispersion instrument since.