What it does
Thomas Young shone one light through two narrow slits close together. On a screen beyond, instead of two bright lines, he saw a row of evenly spaced bright and dark bands, or fringes. Light from the two slits travels different distances to each point; where the path difference is a whole number of wavelengths the waves arrive in step and add up (bright), and where it is a half number they cancel (dark). That was strong evidence that light is a wave.
This tool draws the fringes as they would look on the screen, plots the brightness across it, and works out the fringe width β = λD/d with your numbers. You can put a point P anywhere and read its path difference, switch to white light, or close one slit to see single-slit diffraction. It follows the Class 12 Physics chapter on wave optics (interference, Young’s experiment, diffraction at a single slit). To see why light spreads out of a slit at all, start with huygens wavelets; the same adding of waves on a string is in travelling wave, and a research-level version with gratings and round holes is diffraction pattern generator.
How to use it
- Pick Double slit or Single slit, and One colour or White light.
- Slide the wavelength λ, the slit separation d (double slit only), the slit width a and the distance D to the screen.
- Move point P along the screen. The red marker and the dashed line on the graph show where it is; the facts give the path difference there in wavelengths.
- Tick Also show the other pattern to draw the single-slit curve under the double-slit one (or the other way round).
Worked example
The page opens with red-orange light, λ = 600 nm, slits d = 0.25 mm apart, each a = 0.05 mm wide, and a screen D = 1 m away. Then
β = λD/d = 600 × 10⁻⁹ × 1 / 0.25 × 10⁻³ = 2.4 mm.
The third bright fringe is 3β = 7.2 mm from the centre and the first dark one is at β/2 = 1.2 mm. P starts at 3.6 mm, where the path difference is d x / D = 900 nm = 1.5 λ: a dark fringe.
Switch to a single slit of the same width: the first minima are where a sin θ = λ, so the central maximum is about 2λD/a = 24 mm wide, ten times the double-slit fringe width. In fact the double-slit fringes sit inside this wide envelope, about 2d/a = 10 of them in the central band.
Things to notice
- β grows with λ and D, and shrinks as d grows. That is why the experiment needs slits a fraction of a millimetre apart.
- With white light the central fringe is white (every colour has zero path difference there). The next fringes are coloured, and soon the colours overlap and wash out.
- A single slit’s central maximum is twice as wide as the others and much brighter. Narrow the slit and it spreads more.
Check yourself
Four questions with fresh numbers: a fringe width, a wavelength from the fringe width, the position of the nth bright fringe, and the width of a single slit’s central maximum. Nothing is saved or sent anywhere.
FAQ
Why are the double-slit fringes not all equally bright here? Each slit has a width, so each one also diffracts, and its single-slit pattern acts as an envelope. With very narrow slits all fringes would be equally bright.
Is the fringe width the same for bright and dark fringes? Yes: the distance between two neighbouring bright fringes equals the distance between two neighbouring dark ones, β.
Does the tool use the small-angle approximation? The graph and screen use the exact angle. The formulas in the working use the usual school approximations, which agree to better than 0.1% here.
Where the numbers come from
- Fringe width β = λD/d, bright fringes at x = nβ and dark at (n − ½)β, single-slit minima a sin θ = nλ and the Fraunhofer intensity sinc²·cos²: standard wave optics, as taught in Class 12 physics.
- Wavelength-to-colour mapping: a common piecewise approximation for screens (after Dan Bruton's public description); colours are approximate.
- Checked against an independent Python calculation: β, the minima and the exact path difference with math, and the intensity curve against a numerical Huygens sum of 20 000 point sources across each slit (numpy).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.