C.3 · IB Physics
Double-slit interference
Two coherent sources give bright and dark fringes where the path difference is a whole or a half number of wavelengths. Drag the probe point P along the screen and read the path difference there.
Predict first
Start from the default settings. If you halve the slit separation d from 0.50 mm to 0.25 mm, what happens to the fringe spacing on the screen?
Fringe spacing s = λD ÷ d, so halving d doubles s: 2.4 mm becomes 4.8 mm.
Check it in the model: Move the Slit separation, d slider from 0.50 to 0.25 mm and compare the Fringe spacing readout.
intensity single-slit envelope path to probe P · drag on the screen to move P
dark: Δ = (n + ½)λ
fringe spacing s ≈ λD ÷ d (small angles)
I = I₀ cos²(πd sinθ ÷ λ) × sinc²(πa sinθ ÷ λ)
Try this
- Keep λ = 600 nm and D = 2.0 m. Halve d from 0.50 mm to 0.25 mm and compare the fringe spacing readout with the first value.
- Drag P to a dark fringe, then to the next bright one. Check the path difference changes by exactly one wavelength between neighbouring bright fringes.
- Choose Real slits (envelope). Count the bright fringes in the central envelope. Why is one expected order missing?
Exam tip: bright fringes need path difference nλ, dark fringes (n + ½)λ, and the spacing is s = λD ÷ d, so wider slit separation gives narrower fringes. Write path difference in terms of λ, not as a bare length.