How do diffraction patterns form in a double-slit experiment?

In a double-slit experiment, diffraction patterns form due to the combination of diffraction at each slit and interference between the light waves emerging from the two slits.

Here’s how the pattern forms:

1. Diffraction at individual slits:
   When coherent light (light of the same wavelength and phase) passes through each slit, it spreads out (diffracts) into semicircular wavefronts, just like waves from a point source.
2. Superposition of waves:
   The waves from the two slits overlap in the space beyond the slits. At different points on a screen, these waves combine according to the principle of superposition.
3. Constructive interference (bright fringes):
   Occurs when the path difference between the two waves is an integer multiple of the wavelength (nλ). This causes the waves to reinforce each other, producing bright bands.
4. Destructive interference (dark fringes):
   Occurs when the path difference is a half-integer multiple of the wavelength ((n + ½)λ). The waves cancel each other out, resulting in dark bands.
5. Resulting pattern:
   A series of alternating bright and dark fringes appears on the screen — this is the interference pattern. The central fringe is the brightest, with successive fringes on either side becoming dimmer.

So, diffraction allows the light to spread out from each slit, and interference between the two diffracted waves produces the visible pattern.