YDSE path difference (bright fringes) (Δx = nλ (bright))

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Δx = nλ (bright)

What each symbol means

SymbolWhat it stands for
nOrder of the bright fringe; integer …, −2, −1, 0, 1, 2, … (constructive interference)
λWavelength of light in the medium between slits and screen
ΔxPath difference between the two coherent waves arriving at a point on the screen (same units as λ)

When to use this

Young’s double slit (YDSE); coherent, monochromatic (or narrow-band) sources; same medium for path difference and λ; bright-fringe condition Δx = nλ (plus any extra π phase shifts if a medium flip is specified).