Dispersion is white light splitting into colours because each wavelength meets a slightly different refractive index. Drag the sliders below to set the prism apex angle, angle of incidence, mean refractive index and Abbe number, and watch the emerging spectrum and its exact deviation angles respond in real time.
The four controls map onto the four things that really decide what a prism does. The apex angle A sets how sharply the two refracting faces meet; the angle of incidence i sets how the white beam strikes the first face; the mean index nd says how strongly the glass bends the middle of the spectrum; and the Abbe number Vd says how differently it treats red and violet. Under the canvas, the sim is simply solving Snell's law, sin i = n sin r, at both faces — the same two-boundary refraction you can work through one interface at a time in the Snell's law calculator — with each colour's index set by Cauchy's relation n(λ) = A + B/λ².
The readouts are there to prove the core rule rather than ask you to trust it. At every setting you can reach, n (violet) is larger than n (red), and the violet deviation sits above the red one — the fan on screen always puts violet at the bigger angle. That corrects the classic exam mistake of expecting red, the “stronger” colour, to bend most: bending is set by the index, and the index rises as the wavelength shortens. Since a medium slows light to v = c/n (the idea explored in our speed of light guide), violet is also the colour the glass slows the most.
Two experiments are worth running deliberately. First, drag Vd up and watch the angular-dispersion readout shrink while the whole fan stays put: a high Abbe number means the indices for red and violet crowd together, and the spectrum narrows. Then drag nd up instead and watch every deviation grow at once — the fan swings as a whole. Colour itself is fixed by frequency rather than by the wavelength that changes inside the glass, a distinction unpacked in the frequency formula guide.
Finally, push the geometry until it breaks. Raise A or nd and the status line will announce the moment violet stops emerging: it has hit the second face beyond the critical angle and is totally internally reflected, shown as a dashed ray bouncing back inside the prism. Sweep i slowly from 20° to 80° and each deviation dips through a minimum — the symmetric passage a prism spectrometer exploits to measure the index of a glass sample.
Violet leaves at a bigger angle because the glass refracts short wavelengths more strongly, so its refractive index is higher for violet than for red. A higher index bends the ray harder at both prism faces, giving violet the larger total deviation. The readouts prove it at every slider setting: n(violet) always sits above n(red), and the violet deviation always exceeds the red one.
The Abbe number sets how strongly the glass disperses: a low value means the refractive index changes a lot between red and violet, so the spectrum fans out wide, while a high value means nearly the same index for every colour and a narrow fan. Slide it down from 75 toward 15 and watch the angular-dispersion readout grow while the mean deviation barely moves.
It fits Cauchy's two-term formula, n = A + B/(wavelength squared), using your mean index and Abbe number at the standard 656.3, 587.6 and 486.1 nm reference lines. Evaluating that formula at 680 nm gives n for red and at 410 nm gives n for violet, and each colour is then traced through both faces of the prism with Snell's law using its own index.
Yes — sweep the angle-of-incidence slider slowly and watch a deviation readout fall, bottom out and rise again; the lowest value is the minimum deviation for that colour. It happens at the symmetric setting where the ray crosses the prism parallel to its base, entering and leaving at equal angles. Prism spectrometers measure the refractive index of glass from exactly this angle.
Because it hits the second face steeper than the critical angle and undergoes total internal reflection: the ray reflects back inside the prism instead of refracting out. Violet fails first, since its higher index means a smaller critical angle. The status line flags this the moment it happens — recover the ray by lowering the apex angle or the index, or by raising the angle of incidence.