STUCK IN PHYSICSAll simulations ↗TWO SLITS · DIFFRACTION · INTERFERENCE
Young’s double-slit experiment.
Two slits make the fringes. Their width shapes the envelope. Change either and watch the screen respond.
From the slits to point P
Constructive interference
Plane wavefronts reach the openings; the slits act as coherent secondary sources of circular wavefronts. Dimensions and wave spacing are magnified; the sketch is not to scale.
Path lengths are rounded; Δr is calculated before rounding. The wavelength is in nanometres, path difference in micrometres.
Central maximum: both paths have equal length.
SAME SCREEN. SAME SELECTED POINT.
Intensity against screen position
Time-averaged intensity
Click or drag on the curve to move point P. The dashed line marks its screen position. The screen runs from −15 mm to +15 mm.
Long dash: diffraction envelopeSelected wavelengthDashed: reference colour
Path difference sets the phase
These conditions describe interference between the two slit centres. Slit width also affects how much light reaches each point.
Constructive interference: the two slit-centre contributions arrive in phase. A diffraction minimum can still make this order disappear.
Destructive interference: the two equal slit contributions cancel. Finite slit width also creates its own diffraction minima.
The envelope slightly shifts the positions of the actual bright peaks away from the simple whole-number path condition. The nearest-maximum button finds the peak of the full pattern.
Intensity is shown relative to the central maximum. The path difference Δr is signed; the intensity is the same at +y and −y.
Predict, then check
1. Use very narrow slits. The bright peaks are nearly equal in height, although dark fringes remain between them.
2. Widen the slits while keeping d fixed. The envelope narrows and the outer bright fringes weaken.
3. Increase d. More interference fringes fit inside the same diffraction envelope.
4. Try the missing-order preset. At Δr = 4λ, constructive interference coincides with a diffraction minimum.
Exact path lengths are used. In white-light mode, the solid graph adds the intensities of different wavelengths; the dashed curve shows the selected reference colour.
Why is the centre white?
At the centre, Δr = 0 for every wavelength: all colours reach a maximum together. Away from the centre, maxima for different colours occur at different positions. Coloured fringes appear close to the middle, then overlap and lose contrast farther out. The finite slit width also dims the outer pattern under a diffraction envelope; each wavelength has its own envelope.
A minimum for the selected colour does not cancel all of white light. The white-light graph therefore need not reach zero where the dashed single-colour curve does.
About this model
Two coherent copies of the same illumination reach a screen. Each wavelength interferes only with its matching wavelength from the other source. Different wavelengths are mutually incoherent, so their time-averaged intensities add.
Intensity is normalised to 1 at the centre for every setup. A narrower slit transmits less light in reality; this normalised display compares the shapes of patterns, not their absolute brightness. Making the slits narrow flattens the envelope, not the alternating interference fringes.
Single-colour light covers 380–750 nm. White light uses equal spectral power over that range, sampled at 1 nm. The graph shows total relative radiant intensity, normalised to the central maximum. Approximate CIE colour matching and sRGB conversion provide illustrative screen colours; the white spectrum is white-balanced at the centre. Colours and perceived brightness are display-dependent.
The sketch magnifies source separation and wavefront spacing independently; it is not to scale. White-mode wavefronts show only three representative colours, while the screen and total graph use the full sampled spectrum. The coloured paths identify S₁ and S₂ and do not represent different source wavelengths. Two identical, uniformly illuminated rectangular slits are treated in the far field. Each single-slit diffraction envelope multiplies its two-slit interference pattern. Path lengths are calculated to slit centres; Δr/d supplies the far-field angular coordinate. Geometric spreading and spectral variation in detector response are omitted.
Change the slit width, centre separation, wavelength and screen distance. Compare coloured fringes with their diffraction envelope, select a screen point to inspect the path difference, or try white light and missing orders.
Supports interference and diffraction topics in IB Physics, A Level Physics and AP Physics 2. Two coherently illuminated slit openings act as secondary sources. The model uses a far-field diffraction envelope with exact slit-centre path lengths and normalises intensity to the central maximum.
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