Optics of the eye, accommodation and refractive errors !
- Concept
- Exam priority: medium
The cornea and lens bend light onto the retina; if the eye is too long or too short, the image blurs.
Updated 2026-11-01
Also called: Myopia, Hyperopia, Presbyopia
See it in 3D: Cornea · Lens · Ciliary body · Zonular fibres · Retina · Oculomotor nerve (III)
What it is
The eye focuses light with two lenses: the cornea, which gives most of the power and is fixed, and the crystalline lens, whose power changes for near vision (accommodation). A sharp image needs the focal point to fall exactly on the retina. Refractive errors happen when eye length and optical power do not match.
Key facts
- Light bends most where refractive index changes most: at the air–cornea surface. That is why vision blurs under water.
- Astigmatism: the cornea curves more in one meridian than another; cylindrical lenses correct it.
- A small pupil increases depth of focus (pinhole effect), which is why squinting sharpens vision.
- Cataract (opacity of the lens) scatters light: glare, faded colours and slowly blurred vision.
Equations
- P
- power in diopters (D)
- f
- focal length in metres
A +2 D lens focuses parallel light at 0.5 m.
Approximate optical values (adult)
| Parameter | Value | Note |
|---|---|---|
| Total power of the eye | ≈60 D | relaxed for distance |
| Cornea | ≈40–45 D | fixed |
| Crystalline lens | ≈15–20 D | adjustable |
| Accommodation range | ≈12–14 D in children, under 2 D by about age 60 | declines with age |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Uncorrected refractive error is a leading cause of visual impairment worldwide, and of poor school performance in children.
- Anticholinergic eye drops paralyse accommodation (cycloplegia) and dilate the pupil, used to measure the true refraction in children.
- Cataract is the main cause of reversible blindness; surgery replaces the lens with an artificial one.
Test yourself
Think of the answer, then tap the question to see it.
Why does the lens become rounder when the ciliary muscle contracts?
The ciliary muscle acts like a sphincter: contracting brings it closer to the lens, the zonules slacken, and the elastic capsule lets the lens round up, increasing its power.
A myopic eye focuses distant objects where, and what lens corrects it?
In front of the retina, usually because the eye is too long. A diverging (minus) lens moves the focal point back onto the retina.
Why does a 50-year-old need reading glasses even with no previous refractive error?
Presbyopia: the lens stiffens with age and cannot round up enough to add the power needed for near objects.
Which structure has the largest refractive power in the eye, and why?
The cornea, because the biggest change in refractive index is between air and cornea; the lens is surrounded by fluids of similar index.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The visual pathway, pupillary reflexes and eye movements
- The retina and phototransduction
- Aqueous humor and intraocular pressure
- Sympathetic and parasympathetic effects on each organ
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 14.1 Sensory Perception (OpenStax (Rice University), 2022); Anatomy, Head and Neck, Eye Ciliary Muscles (StatPearls (NCBI Bookshelf), 2024); Cataract (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 50). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.