Young’s double-slit experiment
Interference of light · exact path difference S
1
P − S
2
P · fringe spacing Δy = λD/a
Slits & screen
S1 / S2
point P
bright fringe
dark fringe
drag P up/down the screen — or drag along the dotted guide in the geometry panel
Geometry diagram
Intensity graph
Geometry (exam-style diagram)
Slit sep. a
0.40 mm
Slit width b
0.06 mm
λ (vacuum)
580 nm
Screen dist. D
1.50 m
Violet 450nm
Blue 480nm
Green 530nm
Yellow 580nm
Orange 620nm
Red 660nm
White light
Auto-scan
Pause
Reset
Diffraction envelope
I / I₀ = 1.00 · S₁P − S₂P = 0.00 µm (0.00λ)
path diff = S
1
P − S
2
P ≈ ay/D
bright: S
1
P − S
2
P = nλ
dark: (n+½)λ
fringe spacing Δy = λD/a
P sits on the central bright fringe: the path difference is zero, so every wavelength arrives in phase here — this is the one fringe that stays white even under white light.