Michelson Interferometer

Michelson interferometer — a beamsplitter sends light along two arms and each beam returns before recombination. The signed optical path difference is δ=2(L₁−L₂), so the displayed order is δ/λ. Moving M₂ by λ/2 changes its round trip by λ and therefore moves the pattern by exactly one fringe (the sign shows direction). Use the λ/2 action for this controlled step or Run for an illustrative oscillation. This is a schematic equal-index model; it omits alignment, finite coherence, dispersion and fringe geometry.