In a Michelson interferometer using light of wavelength 546 nm, the movable mirror is shifted until 500 fringes pass the crosswire. (a) How far did the mirror move? (b) If the mirror moves 0.273 mm and 1000 fringes are counted, what is the wavelength? Solution: (a) Moving the mirror by λ/2 shifts one fringe. So distance = N × λ/2 = 500 × 546 nm / 2 = 500 × 273 nm = 136,500 nm = 0.1365 mm. The mirror moved only about 0.14 mm! (b) Rearranging: λ = 2d/N = 2 × 0.273 mm / 1000 = 0.000546 mm = 546 nm. This shows how fringe counting measures wavelengths to high precision.