Eye Refraction — Short Notes

1. Basic Concept of Refraction

Refraction is the bending of light rays when they pass from one transparent medium into another with a different refractive index (RI).

Refractive index = velocity of light in air ÷ velocity in the substance

The degree of bending depends on:

The ratio of refractive indices of the two media
The angle at which light strikes the interface

2. Refracting Media of the Eye

The eye has four refractive interfaces, each between media of differing RI:

Medium	Refractive Index
Air	1.00
Cornea	1.38
Aqueous humor	1.33
Crystalline lens (avg)	1.40
Vitreous humor	1.34

$Lens system of the eye with refractive indices labeled$

Figure: The lens system of the eye with refractive indices. — Guyton & Hall Medical Physiology

The four interfaces are:

Air → anterior surface of cornea
Posterior cornea → aqueous humor
Aqueous humor → anterior lens
Posterior lens → vitreous humor

3. Total Refractive Power

Total refractive power of the eye = 59 diopters (for distant vision)
~2/3 (≈40 D) is contributed by the anterior corneal surface — because the RI difference between air and cornea is the greatest
~1/3 (≈20 D) is contributed by the crystalline lens — which can change its power via accommodation

The "Reduced Eye"

All refracting surfaces can be simplified into one single lens placed 17 mm in front of the retina with 59 D total power. This is called the reduced eye model.

4. Diopter (Unit of Lens Power)

A diopter is the reciprocal of the focal length in meters.

A lens with focal length of 1 meter = +1 diopter
Convex (converging) lenses → positive diopters
Concave (diverging) lenses → negative diopters

5. Accommodation

The crystalline lens adjusts its curvature via ciliary muscle contraction, increasing refractive power to focus on near objects. This is called accommodation.

For distant vision: ciliary muscle relaxed → lens flattened
For near vision: ciliary muscle contracts → lens rounds up → refractive power increases

6. Emmetropia (Normal Refraction)

In emmetropia, parallel rays from infinity are focused exactly on the retina with the ciliary muscle relaxed. This is the ideal/normal refractive state — though enjoyed by only a minority of the population.

7. Refractive Errors (Ametropia)

Emmetropia, hyperopia, and myopia diagram

Figure: Light ray focus in emmetropia, hyperopia, and myopia. — Guyton & Hall Medical Physiology

a) Myopia (Nearsightedness)

Eyeball is too long (or lens too powerful)
Light focuses in front of the retina
Distant objects blurred; near objects clear
Correction: Concave (diverging) lens (negative diopters)

b) Hyperopia / Hypermetropia (Farsightedness)

Eyeball is too short (or lens too weak)
Light focuses behind the retina
Distant objects may be seen by accommodating; near objects blurred
Correction: Convex (converging) lens (positive diopters)

c) Astigmatism

Cornea is not spherical — different curvature in different planes
Light in one plane focuses at a different distance than at right angles
Produces blurred/distorted vision in all distances
Correction: Cylindrical lens oriented at the appropriate axis

d) Presbyopia

Age-related loss of lens elasticity → reduced accommodative power
Emmetropes need reading glasses; myopes may remove distance glasses to read
Appears in middle age (typically after 40 years)

8. Clinical Testing

Test	Use
Snellen chart (6 m / 20 ft)	Visual acuity — normal is 6/6 (20/20)
Pinhole test	If acuity improves through a pinhole → refractive error is the cause
Trial and error lens	Determine strength of corrective lens
Cylinder axis chart	Parallel bars at various angles to identify axis of astigmatism

Tip: If acuity is worse than 6/240 (20/800), record as counting fingers, hand motions, or light perception.

9. Surgical Correction

When glasses/contacts are not preferred:

LASIK (Laser in situ keratomileusis) — permanently reshapes corneal curvature
PRK (Photorefractive keratectomy) — surface laser ablation
Contact lenses — nullify corneal refraction by replacing the air–cornea interface with a tear film of near-equal RI to the cornea; especially useful in keratoconus

Summary Table

Condition	Defect	Focal Point	Correction
Emmetropia	None	On retina	None
Myopia	Long eyeball / strong lens	In front of retina	Concave lens
Hyperopia	Short eyeball / weak lens	Behind retina	Convex lens
Astigmatism	Asymmetric corneal curvature	Different planes	Cylindrical lens
Presbyopia	Reduced lens elasticity	Variable	Reading glasses

Sources: Guyton & Hall Textbook of Medical Physiology, pp. 619–626; Harrison's Principles of Internal Medicine 22E, p. 264

This is a shared conversation. Sign in to Orris to start your own chat.