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Refraction basics
Anatomy & Optics
Key Points
- •Snell's law (n1 sin i = n2 sin r) governs refraction at every optical interface in the eye - the cornea (n=1.376), aqueous (n=1.336), lens (n=1.42 average), and vitreous (n=1.336)
- •The cornea is the most powerful refracting surface (+43 D) because the cornea-air interface has the largest refractive index difference (1.376 − 1.000)
- •Total internal reflection occurs when light passes from a denser to a rarer medium at an angle exceeding the critical angle (≈46° for cornea-air); this is why gonioscopy is needed to view the angle
- •The reduced eye (Listing) simplifies calculations: single +60 D surface, nodal point at 7.08 mm, posterior focal length 24.13 mm
- •The keratometric index (1.3375) used in keratometers differs from the true corneal RI (1.376) - it accounts for the posterior corneal surface; post-LASIK this assumption fails, causing IOL calculation errors
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- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Risk factors and associations