The cornea
The cornea is a highly transparent structure of meniscus from approximately 12 mm in diameter and slightly smaller vertically than horizontally. the center thickness is usually between 0.5 and 0.6 mm.
A thin layer of lacrimal fluid normally covers the anterior surface, but it is too thin to affect the power appreciably and may be ignored in this context.
To first approximation, both surfaces may be regarded as spherical, the radii of curvature having values approximately ➕7.7 mm (anterior) and ➕6.8 mm (posterior)
The refractive index of the corneal substance may be taken as 1.376 and that of the aqueous humour, in contact with the back surface of the cornea, as 1.336 By applying equation F=(n'-n) R, ( where F= power of spherical refracting surface, R = reciprocal of radius of curvature in metres, n= refractive index of first medium and n' = refractive index of second medium), the tow surface power of the cornea may be found as follows:
• Anterior surface
1000(1.376-1)
Power F 1 = ➖➖➖➖➖➖➖➖
+7.7
= +48.83 D
• Posterior surface
1000(1.336-1.376)
Power F 1 = ➖➖➖➖➖➖➖➖
+ 6.8
= -5.88 D
The power of the cornea as a whole is, therefore, about + 43 D, over tow-thirds of the total power of the eye.
The light entering the eye is refracted markedly at anterior corneal surface for tow reasons: first, because of curvature and secondly, because of the big difference in refractive indices of air (1) and cornea (1.37). When the eyes are unprotected under water, the anterior surface of the cornea has its power greatly reduced, the retinal image then becoming inordinately blurred.
The aspherical shape of cornea's anterior surface is responsible for a baseline astigmatism of 0.25 -0.5 D in almost every human eye. Further, a changing corneal surface's radius of curvature with distance from the centre of the pupil to the pupillary margin is responsible for a spherical aberration of 0.21-162 D ( depending on the specific corneal from ) in a normal human eye for a pupil of 4 mm in diameter.r
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