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Double Slit Interference Calculator

Work out double slit interference instantly with clear inputs, formula shown and shareable results.

About Double Slit Interference Calculator

Young's double slit produces evenly spaced fringes with separation lambda.L/d. Helium-neon light at 632.8 nm through 0.25 mm slits, viewed 2 m away, gives 5.06 mm between bright fringes. Closer slits spread the pattern out, which is why the effect is easier to see with fine slits.

Formulas

Fringe spacing

y = lambda L / d

Bright fringe condition

d sin(theta) = m lambda

Worked example

With the default values below, Double Slit Interference Calculator returns Fringe Spacing of 5.0624. Change any input above and every figure here is recalculated in your browser.

Inputs used

Slit separationminimum 0 mm
0.25 mm
Wavelengthminimum 0 nm
632.8 nm
Distance to the screenminimum 0 m
2 m

Results

Fringe Spacing
5.0624 mm
Same Spacing In Metres
5.0624 × 10⁻³
Angle To The First Bright Fringe
0.1450 deg
Fringes Per Centimetre
1.975

Step by step

  1. 1Values used
  2. 2Fringe spacing
  3. 3Bright fringe condition
  4. 4Fringe Spacing= 5.0624 mm
  5. 5Same Spacing In Metres= 0.0051
  6. 6Angle To The First Bright Fringe= 0.1450 deg
  7. 7Fringes Per Centimetre= 1.975

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does the pattern spread when the slits are closer?

Because spacing goes inversely with d. Wide slits crowd the fringes together until they are unresolvable.

What does this experiment demonstrate?

That light interferes, so it behaves as a wave. Repeating it with single electrons is the classic demonstration of quantum superposition.

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