How to use it
Pick one slit, two slits or a grating with up to 20 slits. Light arrives from the left, the black strip shows the screen in a dark room and the graph plots the brightness across it. Move a slider and the pattern changes at once: a longer wavelength or a more distant screen spreads the fringes out, while slits further apart squeeze them together. The top view shows why: wavelets leave every slit, and where crests meet crests the light is bright.
Controls
- Setup and light: single slit, double slit or grating; one wavelength (380–750 nm) or white light.
- Sliders: wavelength λ, slit width a, slit separation d (centre to centre), number of slits N and screen distance L. The width can never exceed the separation.
- Screen view: visible width, Fit to pattern, exposure (like a long photo) and the dashed envelope.
- Measuring: drag marker A or B on the screen or the graph, or click or tap where the nearest one should go. Snap puts a marker on the centre of a bright fringe (for one slit, a dark fringe or the centre). Keyboard: focus the graph, then arrow keys, A, B or Space, Shift for big steps.
- Top view and photons: moving waves or time-averaged brightness; start, pause and clear photons at a chosen rate. Settings are saved in this browser.
How it works
Light from two slits a distance d apart travels different distances to a point on the screen at angle θ. The difference is d sin θ. When it is a whole number of wavelengths, the waves arrive in step and make a bright fringe; halfway between, they cancel. On a screen at distance L the point sits at y = L tan θ:
Each slit has a width a, and light from different parts of one slit interferes too. That gives the envelope, with dark bands where a sin θ = mλ (m ≠ 0). The page plots the Fraunhofer formula for N equal slits, normalised so the central peak is 1:
β = πa sin θ / λ α = πd sin θ / λ
The first factor is the envelope, the second the interference of N slits. More slits make the bright peaks sharper without moving them, with N − 2 weak peaks between them. An order goes missing when it falls on a dark band of the envelope, for example every third order when d = 3a. The formula assumes a distant screen (the far field), so the readouts show a²/(λL), which should be much smaller than 1. The dimming far out on a flat screen is ignored.
In white light every wavelength makes its own pattern. The page adds up to 75 wavelengths from 380 to 750 nm and turns the sum into a colour with the CIE 1931 colour-matching functions (the standard table in 10 nm steps) and the sRGB conversion, balanced so the full mix looks white.
The top view is not to scale: a real wavelength is far too small to draw, so the picture enlarges it, limits d/λ and may show fewer slits (the note under it gives the real numbers). It adds up wavelets from points across every slit, which is Huygens’ principle.
Things to try
- Load “Young's double slit”, measure five fringes with Snap on and compare with λL/d. Then double L.
- Load “Missing orders (d = 3a)” and find the gaps at m = ±3 and ±6.
- In Grating mode raise N from 2 to 20: the peaks get sharper but stay in place.
- Load the grating preset: in white light every order is a small rainbow with violet nearest the centre.
- Start the photons at 2 per second, then speed them up and watch random dots turn into fringes.
Frequently asked questions
Why is the centre white in white light?
At the centre every wavelength has zero path difference, so all colours are bright together. Further out, short wavelengths reach their fringes first, so inner edges look bluish and outer edges reddish.
Does light really arrive one photon at a time?
Yes. The experiment has been done with light so faint that only one photon at a time crosses the apparatus, and with electrons. Each lands at a single spot, yet the dots build the same fringes. The page imitates this with random numbers drawn from the calculated intensity.
How is a grating different from two slits?
A real grating has hundreds of slits per millimetre and thousands in total. Its orders sit at the same angles as for two slits with the same d, but they are much narrower, so colours separate cleanly. That is why gratings are used to measure spectra.
Who first did it?
Thomas Young reported the interference of light to the Royal Society in 1803; his 1807 lectures describe light from two slits. It was key evidence that light is a wave.
Sources
- Young T (1804). The Bakerian Lecture: Experiments and calculations relative to physical optics. Philosophical Transactions of the Royal Society of London 94, 1–16. Young’s 1803 lecture on the interference of light.
- Ling SJ, Sanny J, Moebs W (2016). Double-slit diffraction. University Physics Volume 3, OpenStax, section 4.3. The single-slit envelope (sin β/β)² and missing orders.
- CIE (2019). CIE 1931 colour-matching functions, 2 degree observer (data table). International Commission on Illumination. The table behind the white-light colours.