Young two-slit fringe geometry
For equal-phase coherent illumination of narrow slits separated by d, far-field bright directions satisfy d sinθ=mλ and dark directions d sinθ=(m+½)λ.
Key knowledge
For equal-phase coherent illumination of narrow slits separated by d, far-field bright directions satisfy d sinθ=mλ and dark directions d sinθ=(m+½)λ. In a distant screen's small-angle region, adjacent bright fringes are separated by Δy≈λL/d.
Values and equations
d sinθ=mλ; d sinθ=(m+½)λ; Δy≈λL/d
- d,λ,L,Δy
- Slit separation, wavelength, screen distance and fringe spacing m
- θ
- Viewing angle rad
- m
- Integer fringe label
Conditions and scope
- Common wavelength in the propagation medium, matched polarization, coherent equal-phase slit illumination, narrow slit approximation and L≫d.
- Finite slit width adds a diffraction envelope; source phase offsets move fringes and unequal amplitude prevents completely dark minima.
Sources · 1
References
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Evidence and review
University Physics 3: 3-1-youngs-double-slit-interference: Equations 3.1–3.5 and source/slit coherence assumptions: path difference, angular conditions and screen spacing.
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Entry details
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