When two waves of light travel along the same path, the effect produced depends upon whether or not the waves are in phase with one another.
If they are in phase, the resultant wave will be a summation of the two, and this is called constructive interference.
If the two waves of equal amplitude are out of phase by half a cycle they will cancel each other out called as destructive interference. The final effect in each case is as if the waves were superimposed and added (in the algebraic sense) to each other.
Phase differences of less than half a cycle thus result in a wave of intermediate amplitude and phase Destructive interference occurs within the stroma of the cornea. The collagen bundles of the stroma are so spaced that any light deviated by them is eliminated by destructive interference.
The intensity of the light falling on zone AB is reduced to some extent by interference between the primary and secondary waves. The light falling on zone BC is derived from secondary waves alone and is of much lower intensity.
When light passes through a circular aperture, a circular diffraction pattern is produced. This consists of a bright central disc surrounded by alternate dark and light rings. The central bright zone is known as the Airy disc.
The terms limit of resolution and resolving power refer to the smallest angle of separation between two points which allows the formation of two discernible images by an optical system. The limit of resolution is reached when two Airy discs are separated so that the centre of one falls on the first dark ring of the other.
In the case of lenses and instruments, the diffraction effect at the apertures used is negligible compared with the other errors or aberrations of the system
In the case of the eye, diffraction is the main source of image imperfection when the pupil is small.
Interference phenomena are also utilised in optical instruments. One example is low reflection coatings which are applied to lens surfaces. The coating consists of a thin layer of transparent material of appropriate thickness. Light reflected from the superficial surface of the layer and light reflected from the deep surface of the layer eliminate each other by destructive interference
- Potential Visual Acuity Testing
This tests may be used to assess the potential visual acuity of eyes in which it is impossible to see the macula e.g. because of a cataract. Good potential visual acuity indicates that cataract surgery is likely to be of benefit. The simplest clinical test is the pinhole test
The blue field entoptic phenomenon is the ability to see moving white dots when blue light diffusely illuminates the retina. They are thought to represent light transmitted by white blood cells in the perifoveal capillaries. When this phenomenon is present, macular function is grossly intact.
Interferometers project laser light from two sources on to the retina. Interference occurs where the two sources meet and this is seen as a sine wave grating if the macula is functioning.
The potential acuity meter projects a letter chart on to the retina through a small aperture.
- Correction of Aphakia with an Intra-Ocular Lens
The insertion of an intra-ocular lens (IOL) within the aphakic eye overcomes the optical disadvantages of aphakic spectacles and the handling and wearing difficulties encountered with contact lenses. The IOL becomes part of the optical system of the eye, and because it is situated at or very close to the position of the crystalline lens, problems with RSM do not arise.
Preoperatively, it is desirable to predict the power of the IOL which will render the individual patient emmetropic or, in some cases, produce a desired refractive error.
Many theoretical formulae have been devised for predicting IOL power, based on the calculation of the vergence power required in the plane of the IOL at a known position within the eye.
- Anti-reflective Coatings
The reflection of light from the surface of a lens may be reduced by coating it with a material the thickness of which is a quarter of the wavelength of the incident light. Lights rays which are reflected from the surface of the lens travel a distance of one half of a wavelength further than those which are reflected from the surface of the antireflective coating. This causes destructive interference and reduces the reflection of light).
In contrast, a coating which has a thickness half the wavelength of the incident light produces a mirror coating because of constructive interference. Any wavelength may be selectively reflected by a coating which is half a wavelength thick. Mirror coatings are usually combined with an absorptive tint.
- Optical Coherence Tomography (OCT)
This is an experimental imaging technique analogous to B-scan ultrasound which uses the time delay of infrared light reflected by the retina to provide cross-sectional images of the retina with resolution as small as 10 μm. Light from an infrared source (843 nm) is split into a reference beam which is reflected off a mirror and a sample beam which is reflected off the retina. Temporal differences between the two reflections result in an interference signal which is processed to produce a digital image.
- Laser Interferometry
Interferometers project laser light (usually He–Ne) from two sources on to the retina. Interference occurs where the two beams meet and is seen as a sine wave grating. This effect occurs despite the presence of a cataract or refractive error. Reducing the separation between the light sources reduces the spatial frequency of the sine wave grating and allows the estimation of the potential visual acuity of an eye when the macula cannot be seen because of a cataract.
Reference:: Clinical Optics :Andrew R. Elkington