### diffraction

(effect of an obstacle on a wave)

**Diffraction** is an effect on waves when encountering
an obstacle or opening. For example, when a light wave
hits the edge of something, some of the light is diffracted,
redirecting it around the edge.

Diffraction affects the resolving ability of telescopes,
such that the larger its aperture, the smaller is its
smallest possible object that can be resolved.
A telescope is said to be
diffraction limited if all its other optical
properties are good enough that diffraction is its main
limitation in resolving images.

Diffraction also affects particles as per quantum mechanics'
particle/wave duality.

Much effort has gone into characterizing the effects and
theories and models include:

**Huygens-Fresnel principle**
**Kirchhoff diffraction formula**
**Fresnel diffraction**
**Fraunhofer diffraction**
**Geometrical theory of diffraction**
**uniform theory of diffraction**

**Refraction**, on the other hand, is the change in direction of a light ray
as it passes between two materials with different densities
and is the mechanism of lenses.

(*optics,physics*)
**Further reading:**

http://en.wikipedia.org/wiki/Diffraction

**Referenced by pages:**

Airy disk

angular resolution

aperture

apodization

beam

confusion limit

cosmic dust

diffraction limited

dirty image

diffractive interstellar scintillation (DISS)

electron scattering

Fabry-Pérot interferometer (FPI)

grating

high-resolution imaging

interference

Lyot coronagraph (CLC)

Lyot stop

neutron scattering

Schiefspiegler

seeing

speckles

spherical aberration

Strehl ratio

2D Fourier transform

Index