Interference and Diffraction

(1) Constructive interference:

Phase difference: 2nπ
Path difference (∆): nλ

(2) Destructive interference:
Phase difference: (2m – 1)π
Path difference (∆): (2m – 1)λ/2

(3) $X = {λD}/{d}$ X -> fringe width

(4) $X_{bright} = {nλD}/{d}$

$X_{dark} = {(2m – 1)λD}/{2d}$

(5) Phase difference: ${2πx}/{λ}$