Young's double-slit experiment
Monochromatic light passes through a single slit, which acts as a point source so that the light reaching the double slit is coherent across both slits. Diffraction at each of the two slits provides two coherent sources, and the diffracted beams overlap and interfere, giving evenly spaced bright and dark fringes on a screen a distance D away.
What this actually means
The single slit is the part people cannot justify. Its job is to make both slits receive light from the same wavefront, so the two emerging beams keep a constant phase difference. Without it, different parts of an extended source feed the two slits independently and the pattern washes out.
Diffraction is essential, not incidental. If the slits did not diffract, the two beams would never overlap and there would be nothing to interfere. This is why the slits must be narrow.
A laser makes the single slit unnecessary because the beam is already spatially coherent across the double slit, which is how the experiment is usually done in a school lab now.
The central bright fringe sits on the perpendicular bisector, where both paths are equal, and the fringes on either side correspond to path differences of 1λ, 2λ, 3λ and so on.
The fringes are not all the same brightness. They are modulated by the single-slit diffraction envelope of each individual slit, so outer fringes are dimmer, and any fringe that lands on a diffraction minimum is missing altogether.
Saying the single slit is there to make the light monochromatic. It is there to make the two slits coherent.
Prove it — watch it be true
- Open the computed two-source field and note the anti-nodal lines fanning out from the two slits.
- Read the screen intensity profile and confirm evenly spaced maxima about the central peak.
- Use the coherence-breaking toggle to simulate removing the single slit and watch the fringes disappear.