Diffraction is the tendency of waves to bend, spread and interfere when they meet an obstacle or squeeze through a gap. It is why you can hear someone talking around a corner but cannot see them — sound wavelengths are long enough to wrap around doorways, while light wavelengths are not. The effect shows up in every kind of wave: water ripples curling past a pier, radio signals creeping over hills, and X-rays scattering off crystals.

Why waves do this

The standard explanation is Huygens' principle: every point on a wavefront acts as a source of tiny secondary wavelets, and the new wavefront is the sum of all those ripples. When part of a wavefront gets blocked, the surviving wavelets at the edge propagate into the geometric shadow, curving the pattern inward.^[1] 

Diffraction becomes obvious when the size of the gap or obstacle is comparable to the wavelength. That single rule explains so much everyday experience: AM radio (long wavelengths) hugs terrain far better than 5G signals (short wavelengths), and a large speaker tweeter sounds "beamy" while a woofer fills the room.

Diffraction isn't an exception to straight-line travel. Straight lines are just what waves look like when the obstacles are enormous compared to their wavelength.

Fresnel vs. Fraunhofer

Physicists split diffraction into two regimes based on distance rather than physics:^[2] 

• Fresnel (near-field): Source or screen sits close to the obstacle, so wavefronts are curved. Patterns morph as you move the screen, and the maths involves messy integrals.
• Fraunhofer (far-field): Everything behaves as if at infinity, with plane wavefronts. The pattern keeps its shape as it propagates and the analysis simplifies considerably — often handled with lenses in the lab.

Where it helps instead of hurts

Lens makers fight diffraction because it sets a hard resolution limit: no telescope, however perfectly ground, can separate two points closer than roughly the wavelength divided by the aperture. But other fields treat diffraction as the whole point. X-ray crystallography reads atomic structure directly from diffraction patterns — the method behind the double-helix model of DNA. Diffraction gratings split light into spectra for chemistry and astronomy. Electron diffraction reveals surface structures of materials atom by atom.