|Interference patterns produced by two, closely spaced wave sources in phase. The separation between sources is smaller in the image on the left and larger in the image on the right.|
Nodal lines are hyperbolas. Think about it, the definition of a hyperbola is the locus of all points whose distance to two fixed points have a constant difference. At distances much greater than the wavelength, the nodal lines appear as straight radial lines.
Path length difference
|nλ = Δℓ||(n + ½)λ = Δℓ|
|nλ = d sin θ||(n + ½)λ = d sin θ|
Small angle approximation
n = 0, 1, 2, 3, …
is the order