Double Slit Interference Calculator
Calculate exact geometric bright-fringe position and a far-field two-slit pattern with finite slit-width envelope.
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Result
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Calculation steps
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How to calculate
Combine two equal coherent slit fields and the rectangular single-slit far-field envelope, using exact screen geometry.
sinθm=mλ/d; screen position=L tanθm; normalized I=sinc²(πa sinθ/λ) cos²(πd sinθ/λ)
Worked example
λ=550 nm, slit spacing 0.1 mm and screen distance 2 m give small-angle fringe spacing 11 mm.
Assumptions and input details
Equal uniform coherent slits, width below center spacing, no spectral width or unequal amplitude. Screen distance must be at least 10d²/λ for the scoped far-field approximation. A nominal interference maximum may be suppressed by the diffraction envelope; no laboratory alignment instructions.
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
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The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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