Haidinger–Michelson rings in white light

We use coherence theory to explain why it is necessary modify the conventional setup of a Michelson interferometer obtain Haidinger rings with an extended source white light. The modification consists introducing glass slide into one two arms interferometer. This insertion circumvents drastic restriction imposed by low temporal light, which prevents observation interference setup. In order understand this restriction, we developed and implemented criterion for observing fringes. modified also makes possible perform spectral interferometry experiment analyze output determine refractive index over whole visible spectrum. fit measured data using Sellmeier's law gives extrapolated value IR $n_\mathrm{IR}=1.5307\pm0.0003$ characteristic wavelength $\lambda_0=(164.5\pm0.4)\;\mathrm{nm}$ oscillator responsible dispersion.