Physic Labs

Optics

Dispersion of light and spectra

Split a white-light beam through a prism and verify normal dispersion n(λ)n(\lambda) decreasing with λ: violet deviates most, red least. Adjust the incidence angle and the dispersion strength to compare the paths of the colored rays.

High school

Equipment

  • Virtual prism splitting a white beam into red, green, blue, violet
  • Incidence-angle slider
  • Dispersion-strength slider
  • 'Reset parameters' button and readout n(violet)>n(red)

Procedure

  1. Observe the beam splitting

    Keep dispersion at a medium setting: the white ray entering the prism fans into colors at the exit face — violet bends most toward the base because glass has nviolet>nredn_{violet}>n_{red}. Read the 'normal dispersion' line in the readout.

  2. Vary the incidence angle

    Drag the incidence slider from small toward the limit: every ray's deviation grows near the prism's minimum-deviation angle, and the color fan widens. Note the incidence where the emergent ray nearly grazes the second face.

  3. Change dispersion and compare

    Pull the dispersion slider to 0%: all colors trace one path — a 'non-dispersive' material has λ-independent n. Raise it to 100%: the red–violet separation is maximal. Press 'Reset parameters' and predict the color order before looking again.

Simulation

Experiment history

In 1666, while Cambridge was closed for plague, Isaac Newton passed a sunbeam through a prism and showed white light 'contains' the colors rather than being tinted by the glass — his experimentum crucis recombined the separated colors back into white. Published in Philosophical Transactions in 1672. The wavelength-dependence of refractive index was put into formula by Cauchy in 1836 (n≈A+B/λ2+…n\approx A+B/\lambda^2+\dots) and generalized by Sellmeier in 1871. The Bunsen–Kirchhoff prism spectroscope (1859) turned dispersion into an elemental analysis tool — discovering helium on the Sun before it was found on Earth.

Related physicists

Isaac NewtonJames Clerk Maxwell

Related library topics

Dispersion of light and spectra