Eye structure, accommodation and common defects
Homeostasis and response • The human nervous system
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Basic anatomy and diagram labels
The main external and internal parts used for diagram labelling are sclera, cornea, iris, pupil, lens, ciliary muscles, suspensory ligaments, retina and optic nerve. The sclera forms the white outer layer; the cornea is the transparent front surface. The lens sits behind the pupil and is attached by suspensory ligaments to the ciliary muscles. The retina lines the rear of the eye and connects to the brain via the optic nerve. Diagrams show relative positions and are used to identify each structure for function mapping .
Sclera and cornea: structure → function
The sclera is tough, fibrous and opaque, which protects internal structures and maintains eyeball shape. The cornea is transparent and highly curved, which refracts (bends) incoming light toward the lens. Corneal curvature determines a major proportion of the eye’s focusing power; any change in shape alters where rays meet relative to the retina .
Iris and pupil: controlling light entry
The iris is a ring of muscle with adjustable diameter. Contraction of circular or radial iris muscles changes pupil size. Pupil constriction reduces light entry in bright conditions; pupil dilation increases light entry in dim conditions. Pupil size changes cause corresponding changes in image brightness and depth of field, protecting the retina from excess light and aiding vision in low light .
Lens, ciliary muscles and suspensory ligaments: accommodation mechanism
Accommodation is the change of lens shape to focus on objects at different distances. Ciliary muscles alter tension in suspensory ligaments. When ciliary muscles relax, suspensory ligaments pull tight, causing the lens to become thinner and less convex for distant focus. When ciliary muscles contract, suspensory ligaments loosen, allowing the lens to become thicker and more convex for near focus. The lens therefore adjusts focal length to place an image on the retina .
Retina, rods and cones: detecting light and colour
The retina contains two major receptor types: rods and cones. Rods have high sensitivity and function in low light, enabling vision at dawn/dusk but not colour discrimination. Cones require brighter light and provide colour vision and high-acuity central vision. Receptor cells transduce light into electrical impulses that travel along the optic nerve to visual centres in the brain .
Optic nerve and signal transmission
The optic nerve bundles axons from retinal ganglion cells and transmits encoded visual information to the brain. The nerve carries spatial and intensity information generated by receptors; any damage to the optic nerve causes loss of visual information regardless of optical focus .
Accommodation for distant objects (distant focus)
Cause: Ciliary muscles relax. Effect: Suspensory ligaments become taut and pull on the lens, making the lens thinner and less curved. Result: The lens decreases optical power and focuses parallel rays from distant objects on the retina. The far point of a normal eye is effectively infinity when accommodation is relaxed .
Accommodation for near objects (near focus)
Cause: Ciliary muscles contract. Effect: Suspensory ligaments slacken and allow the lens to adopt a thicker, more convex shape. Result: The lens increases optical power and focuses diverging rays from near objects on the retina. The near point defines the closest distance the eye can focus using accommodation .
Adaptation to dim light
Cause: Low ambient light. Effect: Pupil dilates to admit more light; rods dominate receptor input because rods have greater sensitivity to low intensities. Result: Improved light detection at the cost of reduced colour discrimination and lower acuity, producing greyscale vision in dim conditions .
Myopia and hyperopia: causes and optical effects
Myopia (short-sightedness) occurs when the eye’s optical power is too strong or the eyeball is too long, causing parallel rays from distant objects to focus in front of the retina and producing blurred distant vision. Hyperopia (long-sightedness) occurs when the optical power is too weak or the eyeball is too short, causing rays from near objects to focus behind the retina and producing blurred near vision. Both defects represent incorrect image placement relative to the retina and are corrected by lenses that alter incoming refraction .
Correction and ray diagrams
Cause: Misplaced focus relative to retina. Effect: Spectacle or contact lenses change the convergence/divergence of rays before they enter the eye so that the eye’s optical system focuses light onto the retina. Diverging (concave) lenses spread rays to move the focus backward for myopia. Converging (convex) lenses bend rays inward to move the focus forward for hyperopia. Ray diagrams show incident rays, refraction at corrective lens, and final focus on the retina to visualise correction .
Key notes
Important points to keep in mind