Young's Double Slit Experiment
The classic experiment demonstrating wave-particle duality and wave nature of light via interference fringes produced by coherent light passing through two narrow parallel slits.
Governing formula
y_m = m λ D / d (Bright), y_m = (m + ½) λ D / d (Dark)
SI unit
m
In depth
Demonstrated that light undergoes wave interference. Fringe spacing $beta = λ D / d$ allows direct measurement of light wavelength $λ$.
Examples in the real world
Monochromatic red laser light forming alternating red and dark interference stripes across a screen 2 meters behind double slits.