8.3.4—Λ = ax / D for double-slit interference using light

For slit separation a and screen distance D, the fringe spacing x satisfies λ=ax/D in the small-angle approximation.

Use consistent units and identify x as adjacent bright-fringe spacing, not distance from the centre unless the count is included.

With a=0.25 mm, D=2.0 m and x=5.0 mm, λ=6.25×10⁻⁷ m.

Increasing slit separation reduces fringe spacing; increasing screen distance or wavelength increases it.