Wave optics

The path difference of two coherent waves
Δ_d = d2-d1

The path difference of two coherent waves: interference maximum
Δ_d = k * λ

The path difference of two coherent waves: interference minimum
Δ_d = (2k+1) * λ/2

Thin-film interference: constructive (maximum)
2h n cos(β) = (2k+1) * λ/2

Thin-film interference: destructive (minimum)
2h n cos(β) = k  λ

Radii of Newton's rings
r = saknis(k*R*λ)

Radii of Newton's rings
r = saknis((2k+1)*R*λ / 2)

Light diffraction
l = d^2 / (4*λ)

Diffraction grating: maximums (bright stripes)
d sin(φ) = k*λ

Diffraction grating: minimums (dark stripes)
d sin(φ) = (2k+1)*λ/2