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Double-slit fringe spacing calculator

A-level Waves

The spacing of bright fringes in Young’s double-slit experiment.

w = λ D / s

This free calculator is for A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

wFringe spacing to find 0.00260 m

λWavelength

DDistance from the slits to the screen

sSlit separation

Significant figures

Working

Write the equation
w = λ D / s
Substitute the values
w = (6.5 × 10⁻⁷) × 2 ÷ (5 × 10⁻⁴)
Calculate
w = 0.0026 m
Give the answer to 3 significant figures
w = 0.00260 m
In standard form: 2.60 × 10⁻³ m

Worked example

Find w, the fringe spacing. Given: wavelength λ = 6.50 × 10⁻⁷ m; distance from the slits to the screen D = 2.00 m; slit separation s = 5.00 × 10⁻⁴ m.

Write the equation
w = λ D / s
Substitute the values
w = (6.5 × 10⁻⁷) × 2 ÷ (5 × 10⁻⁴)
Calculate
w = 0.0026 m
Give the answer to 3 significant figures
w = 0.00260 m
In standard form: 2.60 × 10⁻³ m

Symbols and units

Symbol	Quantity	SI unit
w	Fringe spacing	m
λ	Wavelength	m
D	Distance from the slits to the screen	m
s	Slit separation	m

Double-slit fringe spacing: rearranged

To find fringe spacing: w = λ D / s
To find wavelength: λ = w s / D
To find distance from the slits to the screen: D = w s / λ
To find slit separation: s = λ D / w

Exam tip

This approximation works when the angles are small, which means D is much bigger than s.

Common questions

What is the formula for double-slit fringe spacing?

The equation is w = λ D / s, where w is the fringe spacing (m); λ is the wavelength (m); D is the distance from the slits to the screen (m); s is the slit separation (m).

What units should I use for double-slit fringe spacing?

In SI units: fringe spacing in m, wavelength in m, distance from the slits to the screen in m, slit separation in m. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange w = λ D / s?

Rearranging gives these forms: to find fringe spacing: w = λ D / s. to find wavelength: λ = w s / D. to find distance from the slits to the screen: D = w s / λ. to find slit separation: s = λ D / w.

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Read the full explanation on the Double-slit fringe spacing formula page. More waves equations: Wave speed, Frequency and time period, Magnification, Refractive index, Snell’s law, Critical angle, Diffraction grating, Intensity. See the full list on the a-level physics equation calculator page.