Optical · Experiment

Dispersion & the Spectrum

White light is a mixture of every colour. A glass prism bends each colour by a slightly different amount — violet most, red least — so the beam fans out into a rainbow cast on the screen.

Controls

n (red, 700 nm)1.500
n (violet, 400 nm)1.550
Deviation · red—
Deviation · violet—
Angular spread—
Violet bends most, red least — n falls with wavelength, fanning white light into a spectrum.
About this experiment ›

What you are looking at

A single beam of white light enters the left face of a triangular glass prism. White light is really a blend of all visible wavelengths, from deep red (~700 nm) to violet (~400 nm). Inside the glass each wavelength travels at a slightly different speed, so each one bends by a different amount. The beam crosses the prism, refracts a second time leaving the right face, and emerges as a fanned-out band of pure colours — a spectrum — which you can catch on the screen at the right.

Why the colours separate — dispersion

Refraction is governed by Snell's law, where a larger refractive index n bends light more strongly:
n₁ sin θ₁ = n₂ sin θ₂
The trick is that n is not a single number — it depends on wavelength, n(λ). For glass, shorter wavelengths (violet, blue) have a slightly higher index than longer ones (red). This wavelength-dependence of the refractive index is called dispersion. A common model is Cauchy's equation:
n(λ) = A + B / λ²
Because violet sees a larger n, it bends the most at each surface; red sees the smallest n and bends the least. Two refractions (entering and leaving) compound the difference, so the colours leave the prism spread across a few degrees. The dispersion strength slider changes how much n varies with colour — turn it up to exaggerate the rainbow.

The deviation angle

The total bend between the incoming and outgoing beam is the deviation δ. It depends on the angle of incidence, the prism's apex angle, and n. For a given prism there is an angle of incidence that gives a minimum deviation, where the light passes symmetrically and the inside ray runs parallel to the base — this is the configuration used to measure n precisely. Try sweeping the angle of incidence and watch the deviation values dip and rise.

Things to try

Sweep the angle of incidence and watch the spectrum swing and the deviation change. Widen or narrow the apex angle — a fatter prism spreads the colours further. Push the dispersion strength up to see an exaggerated rainbow, or down toward zero where all colours nearly overlap (an ideal non-dispersive material). This is exactly Newton's 1666 experiment that showed white light is composed of colours, and the same physics behind rainbows, where raindrops act as tiny prisms.