Michelson interferometer: fringe visibility and coherence

What you are seeing: the detector intensity of a Michelson interferometer as one mirror moves, producing path difference $L = 2d$. The intensity oscillates as $I(L) = \tfrac12 (1 + V(L) \cos(2 \pi L / \lambda))$, where the visibility $V(L) = e^{-(L/L_c)^2}$ falls off over a coherence length $L_c$. Fringes are sharp near $L = 0$ and wash out as $L \gg L_c$.

A laser ($L_c \sim$ kilometers) maintains visibility over enormous paths; sunlight ($L_c \sim 0.4$ um) decoheres after a few fringes. The readout reports the implied spectral linewidth $\Delta \nu \approx 0.44 c / L_c$.