143. Image-forming Mechanism – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Clear vision depends on refraction, accommodation of the lens, and appropriate pupil size. The cornea and lens refract light so that a focused image falls on the retina.
• Refraction is the change in direction of light when it passes between media having different refractive indices. Light bends towards the normal when it enters a medium of higher refractive index and away from the normal when it enters a medium of lower refractive index.
• The refractive index of the cornea is about 1.376, while that of aqueous and vitreous humor is about 1.336. The total refractive power of the normal eye is approximately 60 diopters. The cornea contributes about 43 diopters and the relaxed lens about 17 diopters.
• The reduced eye is a simplified optical model in which major refraction is assumed to occur at the anterior corneal surface. Its effective focal length is about 17 millimetres and its refractive power is approximately 59–60 diopters.
• The retinal image is real, inverted and diminished. The upper part of an object is projected onto the lower retina, and the left side is projected onto the right retina. The brain interprets the image as upright.
• Accommodation is the increase in lens refractive power required to focus near objects. During distance vision, the ciliary muscle is relaxed, zonular fibres are tense and the lens is relatively flat. During near vision, parasympathetic stimulation contracts the ciliary muscle, relaxes the zonular fibres and allows the elastic lens to become more convex.
• The near point is the closest distance at which an object can be seen clearly with maximum accommodation. It gradually recedes with age because the lens becomes less elastic. This age-related reduction in accommodation is called presbyopia.
• The near response has three components: accommodation, convergence of the visual axes and pupillary constriction. These three changes occur together when shifting gaze from a distant to a near object.
• The pupil is about 2 millimetres in bright light and may dilate to about 8 millimetres in dim light. A pupil of about 2–3 millimetres gives a good balance between brightness and optical clarity and reduces spherical and chromatic aberrations.
• Emmetropia is the normal refractive state in which parallel rays from distant objects focus on the retina without accommodation. Ametropia refers to refractive errors in which light does not focus correctly on the retina.
• Myopia occurs when distant rays focus in front of the retina, commonly because of an elongated eyeball. It causes poor distance vision and is corrected with a concave lens.
• Hypermetropia occurs when rays focus behind the retina, usually because the eyeball is too short or refractive power is insufficient. It is corrected with a convex lens.
• Astigmatism results from unequal curvature of the cornea or lens in different meridians. It causes blurred or distorted vision and is corrected with a cylindrical or toric lens.
• Presbyopia results from reduced lens elasticity with age and causes difficulty with near vision. It is corrected with a convex reading lens; bifocal, trifocal or progressive lenses may also be used.
• Must-know for examinations: Mechanisms of image formation, refractive power of the eye, accommodation and its mechanism, near point, near response, pupil changes, emmetropia, myopia and its correction, hypermetropia and its correction, astigmatism and its correction, and presbyopia and its correction are the major short-question and viva topics identified in the chapter.