Light diffraction is an optical phenomenon related to the wave properties of light: the effects of diffraction can be explained by geometrical optics. Diffraction occurs when light waves pass through a pupil or encounter an obstacle. At the edges of these light waves are "diffractées", issued in directions that geometrical optics and refraction laws could not only predict. Diffraction phenomena of constructive and destructive interference between light waves-based, and is in fact 'wavelength dependent.
The phenomena of diffraction of visible light can be observed under certain conditions, when a beam of white light is reflected by a structure with repetitive patterns, which to diffract the light, separate trains of different wavelength.
Taking into account diffraction is important for an in-depth study of the refractive properties of the eye, because the diffraction impose a limit on the resolving power of eye (Visual acuity). The effects of diffraction can be used to design specific lenses for implants used in ophthalmology, such as Diffractive optics used for cataract surgery (ex: bifocal or trifocal Diffractive implant for cataract surgery). Light diffraction is also involved in some rare Visual phenomena observed after LASIK)Rainbow glare), or after surgery of cataract)light streaks)
Diffraction and interference
Diffraction is characterized by a deviation of the light rays from the direction of travel under the laws of geometrical optics. Diffraction occurs whenever a light wave front encounters an obstacle that is not completely transparent: it stems from the wave aspect of light, and can be interpreted as a phenomenon of interference interesting an infinite number of waves. To understand diffraction, remind some principles of the wavelike design of light, the first plan of which the principle of Huygens.
Light re-emitted to the same properties if it had been issued by a single point source, which then acquires a certain spatial coherence (the waves emitted from that source are "in phase" on a receiver or remote obstacle). This property has been used in the famous experiment say 'slots' (or 'holes') of Young that marks an important step in thehistory of the study of light. The arrival of waves in phase (after diffraction by a first hole) at the level of the slots drilled screen allowed the visibility of interference produced beyond the slots.
The following figure shows the directions that occur builders interference (orders of diffraction) and destructive.
Geometric explanation of interference (Young's experience)
The origin of the phenomena observed is therefore due to the occurrence of interference between waves diffractées through the holes.
Diffraction and eye
With respect to the eye, the edge of the IRIS Pupil is the main source of diffraction for light that contributes to the formation of the retinal image: physiologically, the effects of diffraction are negligible as long as the pupillary diameter is greater than 2 mm approximately. Below one millimeter, pupillary diffraction results in degradation of the retinal image; However, she has as a corollary an increase in depth of field. The placement of an implant Kamra (neo pupil of 1.6 mm) allows to increase the depth of field of the eye, without inducing major degradation of the quality of the retinal image.
Diffraction destroyed the harsh stigma of geometrical optics, an optically perfect eye has a rigorous stigma; the image of a point is a point where meet all the rays spread through the eye in wave optics, diffraction limits the quality of the optics of the eye, and it even more than the diameter of the pupil is small (myosis). In the absence of optical aberrations, diffraction causes an enlargement of the retinal focal task, which becomes wider than the image of a point source infinitesimal. So, the image of a point is never a point; but a task that matches a bright 'sprawl'. In addition to the essential diffraction, this spread can be accentuated by the presence of optical aberrations. There is a function that allows the representation of the spreading of the luminous energy corresponding to the image of a point source on the retina: (FEP) point spread function, most commonly designated by the acronym PSF (Point Spread Function).
When the eye is devoid of optical aberrations (theoretical situation), only diffraction reduced the stigma. The focal task has a task of Airy aspect: a bright central peak surrounded by darker rings. We can predict the diameter of the central peak, using a bit of geometry and a relatively simple reasoning. To simplify, we can represent no not the pupil as a drive but as a crack, in 2 dimensions.
We understand intuitively that when light encounters a repetitive pattern constituted obstacles, or repeated patterns whose dimensions are relatively similar to those of the wavelength, Interferential phenomena will give rise to a particular distribution of light energy rolling (or reflected). This is being used for the realization of the diffractive multifocal implants (which markets measure some microns): for a bifocal implant, the spacing of the markets is calculated so that the light diffracted in order 1 is directed to a household corresponding to an addition useful for vision closely (additional vergence: ex: + 3D). The design and the height of the diffractive steps are to control the distribution of energy between the orders of diffraction. The design of a trifocal diffraction pattern is based on the same principles, applied in a more subtle way.
We realize that the wavelength influences the direction of diffraction. It is more important for the long wavelengths (red) for the short (blue). Thus, diffraction induces a deviation from the color opposite to that of refraction (these are the short wavelengths that are most deflected).
In everyday life, we can observe the effects of diffraction as reflections iridescent engraved face of a compact disc, which are of a similar phenomenon: angles according to which there is colors correspond to angles of maximum diffraction by the diffraction network (formed in by the estate of hollows and bumps) for each wavelength. the light that illuminates the disc must of course be polychromatic (white light).
The diffraction influence the presence of aberration of high degree on the quality of the retinal image, at least for the current pupil diameters (diffraction predominates for less than 2 mm in diameter). The calculation of the Point Spread Function (the point spread function) takes into account diffraction. In addition to allow the calculation of the maximum theoretical resolution of optical systems (including the eye), the study of the PSF and its use to simulate the retinal image of a more complex than a simple point motif are particularly important in certain clinical circumstances.