The eye can be considered optically as a positive double-lens arrangement that casts a real image on a light-sensitive surface, the retina. The lens system comprises:
The anterior corneal surface (cornea-air interface), with + 43 dioptres (D). It has a fixed focal length.
The anterior and posterior surfaces of the lens, with + 15 D in the accommodated eye.
The iris controls light entry to the retina and reduces intra-ocular light scatter.
The lens has the following properties:
Variation in refractive index from the inner core (lower) to the less dense cortex (higher), constituting a gradient-index system (gradient in the index of refraction).
An adjustable focal length to allow for focussing on objects at different distances. This accommodation is achieved by alteration in shape of the lens,
Lengthening of the antero-posterior axis of the eye;
Corrected by placement of a diverging or negative lens in front of the eye.
Shortening of the antero-posterior axis of the eye;
Corrected by placing a converging or positive lens in front of the eye.
Due to uneven curvature of the cornea;
Different focal lengths for light rays striking the eye at different planes; Corrected by cylindrical or sphero-cylindrical (toric) lenses.
produced by the ciliary muscles attaching to the peripheral suspensory ligament. Ciliary muscle relaxation causes flattening of the lens, increasing its focal length. Focussing on a closer object is achieved by ciliary muscle contraction. The closest point on which the eye can focus is the near point. The amplitude of accommodation decreases with age (presbyopia). Overall, the refractive power of the eye depends on:
The refractive power of the cornea, which in turn depends upon its shape;
The depth of the anterior chamber;
The refractive power of the crystalline lens;
The axial length of the eye. The eye consists of three compartments:
The anterior chamber. This contains aqueous humour, which is secreted by the ciliary body into the posterior chamber (between the iris and the lens) and flows through the pupil into the anterior chamber. The fluid is absorbed into the trabecular meshwork, the uveoscleral system and into the episcleral blood vessels. The normal intra-ocular pressure is 10-21 mm Hg.
The posterior chamber.
There is a protective tear film in front of the eye, comprising an oily surface layer, an aqueous layer and a deep mucoid layer. The pre-corneal tear film has the following properties:
Lubrication of the eyelids;
Smoothening of the optical surface;
Nutrient transfer to the corneal epithelium;
Dilution and removal of irritants;
Antibacterial activity: secretory IgA.
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