Young’s Fringes. A single monochromatic point source . Split the light front into two sub-fronts to get two coherent sources. One can do this by two parallel, narrow slits.
d sinθ = n (n=0, 1, 2,…)
It is equal to the wavelength multiplied by an amplification number
for d=1mm, D=1m
D/d = 1000!