Foundations of Optics TERMS (applied and visual)

Interference by a wavefront

Along a given wavefront produced by a monochromatic point source, the wavefront phase is constant. If two parts of this wavefront are selected and then redirected to a common volume in space, interference will result. This is the basis for interference by wayefront diyision.

Knapps rule

Ammetropia must be axial. The power is assumed to stay at +60.00 D while the length will change Fe=Fo Correcting lens must be placed at the anterior focus of the eye d=fo=fe therefore shape factor = +1 If spectacle point is in the eyes anterior focus plane x=o relative spectacle magnification =1 for all degrees of axial ammetropia.

Anisometropia

Difference in the refractive state of the right and left wye

Refractive anisometropia

Differences in the corneal refractive power but both eyes have the same axial power.

Huygens Principle

Each point on a wave front can be considered to be a source of secondary spherical wavelets. The assumption that the actual field at any point beyond the wavefront is the super position of all these wavelets taking into account the amplitude and phase. This explains why waves 'bend' round a corner

The difference between polarised and unpolarised light

For unpolarised light the electric vector is found ossicalating in all possible orientations whereas for linearly polarised light the electric vector oscillates in one direction only.

Far point

Is a point in space conjugate with the retina when the accommodation is fully relaxed. In emmetropic eyes the far point is to infinity.

Describe the diffferences between Emsley-rabbets simplified eye and the reduced eye

Reduced eye- axial length of 22.22mm, radius of curvature power of +60D, refractive index 4/3, much easier to detect movement in the eye Simplified eye - 3 surfaces single surface cornea, anterior lens posterior lens. Used during accommodation when lens power changes focus from far to near.

Simple hypermetropic astigmatism

The retina is situated at the first focal line

Aphakia

There is an absence of the crystalline lens in the optical system of the eye

Aniseikonia

There is an inequality in the perceived size of image in both eyes

The pinhole Test

Uses an opaque disc with a small (1-2mm) hole in the centre. The pinhole will make a blurred retinal image look clearer by reducing the blur circle size. It improves vision by reducing the retinal blur circle. If the patient has really good vision it improves it bu increasing the size of the airy disc and reduces the illuminations of the retinal image. If the vision is not so good the vision then improves it by reducing the size of the retinal blur circle.

Mixed Astigmatism

When one focal line is situated in front of the retina and one focal line is situated behind the retina

Purkinje Images

1- convex surface- virtual and erect image formed behind the reflecting surface; far objects generally at level of lens 2- convex surface- virtual and erect image, posterior to 1 3- convex surface- virtual and erect image, formed in the vitreous and is larger than 1 4- concave surface- real and inverted image formed within the crystalline lens

Spectacle Magnification

As the ratio between the retinal image size in a corrected eye and the basic retinal image size in an uncorected eye

Relative Spectacle Maginification

As the ratio of the retinal image size in the corrected eye to the retinal image size in a specified emmetropic schematic eye.

Axial anisopetropia

Both eyes have the same power but the axial lengths are different

Compound myopic astigmatism

Both focal points lie in front of the retina

Explain the physical reason why a coloured patter is observed when white light is incident on the surface of the puddle.

Light is reflected from the air- petrol interface and from the petrol interface and from the water interface. When these two days of lights meet again then the resultant is the superposition of the two waves resulting in an interference pattern. Different colours are observed due to the varying ————- of the petrol on flown on the water surface.

Explain division of amplitude and the properties needed to create this

Light requires being coherent in order to observe an interference pattern. Division of amplitude is where the amplitude of the coherent light is split into two components which are brought back together again to form an interference pattern.

Explain the difference between linearly and circularly polarised light

Linearly polarised light is where the electric vector ossilates in one direction only. Circularly polarised light is where the electric vector rotates clockwise or counter clockwise we the light travels through the medium.

requirements for a good coating material

Must be tough- scratch resistant, transparent and colourless

Antimatropia

One eye is myopic and the one eye is hypermyopic