VCE · Topic explainer
Calculating double-slit fringe spacing
For a distant screen and small angles, adjacent bright fringes are separated by approximately λL/d. Here λ is the wavelength, L is the distance to the screen and d is the slit separation. Convert lengths to consistent units before substituting.
Worked example
Light of wavelength 600 nm passes through slits 0.300 mm apart. Put the screen 1.50 m away. Convert λ to 6.00 × 10⁻⁷ m and d to 3.00 × 10⁻⁴ m.
The spacing is (6.00 × 10⁻⁷ × 1.50) ÷ (3.00 × 10⁻⁴) = 3.00 × 10⁻³ m, or 3.00 mm. Larger slit separation gives smaller fringe spacing when wavelength and screen distance are fixed.
Official exam practice
Find other years and marking guidance in the Physics official exam archive.