Vision: Light, Refraction, Accommodation & Vision Problems

Vision

The perception of light reflected by various objects.

Electromagnetic radiation

A form of energy with different wavelengths measured in nanometers (nm).

Visible light

The range of electromagnetic wavelengths that the human eye can detect and perceive as colors.

Photon

Basic unit of light that stimulates photoreceptors in the retina.

Refraction

Bending of light as it passes through a translucent object.

Refractive index

Measurement of the amount of refraction an object provides.

Refractive index of air

Approximately 1.0, so light is not significantly bent when traveling through air.

Water and refraction

Water has a greater refractive index than air and therefore bends light more.

Effect of angle on refraction

The amount of refraction depends partly on the angle at which light strikes a surface.

Refraction through a curved surface

Light striking the edge of a curved surface is bent more than light striking the middle.

Convex lens

Lens with surfaces that bulge outward and cause light rays to bend inward.

Convergence

Bending of light rays inward toward one another.

Focal point

Point at which converging light rays meet.

Effect of a convex lens

Causes light rays to converge on a focal point.

Concave lens

Lens with a surface that caves inward and causes light rays to bend outward.

Divergence

Bending of light rays outward and away from one another.

Effect of a concave lens

Causes light rays to diverge so they do not focus.

Requirement for clear vision

Light rays must be focused at a focal point on the retina.

Why the eye must refract light

Light reflected from nontransparent objects scatters in a divergent pattern and must be refracted to focus on the retina.

Main refractive structure of the eye

The cornea provides about two-thirds of the eye's refractive power.

Role of the lens in refraction

The lens provides fine refractive adjustments needed to focus light on the retina.

Light rays from distant objects

Rays from objects 20 feet or more away are nearly parallel when they reach the eye.

Lens shape for distant vision

The lens remains in its normal flattened shape because distant light rays require little additional refraction.

Emmetropic state

Relaxed state of the eye during distance vision when the lens is flattened.

Light rays from nearby objects

Light rays from nearby objects are more divergent and require greater refraction.

Accommodation

Process in which the lens rounds up to provide greater refraction for near vision.

Ciliary body during accommodation

The smooth muscle of the ciliary body contracts.

Effect of ciliary body contraction

The ciliary body moves closer to the lens.

Suspensory ligaments during accommodation

Tension on the suspensory ligaments decreases and they loosen.

Lens during accommodation

Elastic fibers allow the lens to round up into a more spherical shape.

Result of a rounded lens

A rounded lens refracts light more strongly, allowing near objects to focus on the retina.

Ciliary body during distant vision

The smooth muscle of the ciliary body relaxes.

Suspensory ligaments during distant vision

When the ciliary body relaxes, it moves farther from the lens and pulls on the suspensory ligaments, causing them to tighten.

Lens during distant vision

Tight suspensory ligaments flatten the lens.

Near vision sequence

Ciliary muscle contracts → ciliary body moves closer to lens → suspensory ligaments loosen → lens rounds up → refraction increases.

Distance vision sequence

Ciliary muscle relaxes → ciliary body moves away from lens → suspensory ligaments tighten → lens flattens → less refraction is needed.

Three processes involved in focusing on near objects

Accommodation, pupillary constriction, and convergence.

Pupillary constriction during near vision

Pupils constrict slightly to cover the edge of the lens.

Purpose of pupillary constriction during near vision

Prevents light rays entering the edge of the lens from producing blurry images on the retina.

Convergence of the eyes

Both eyeballs move medially so a near object can be focused on the fovea of each eye.

Near point of accommodation

Closest point at which a person can focus clearly on an object.

Effect of aging on the near point of accommodation

The near point increases with age as the lens stiffens and becomes less able to accommodate.

Presbyopia

Difficulty with reading or other activities requiring close vision due to reduced accommodation with age.

Near point in presbyopia by the mid-40s

Approximately 10–20 inches according to the PowerPoint.

Correction for presbyopia

Reading glasses or bifocals.

Hyperopia

Farsighted refractive error in which the eyeball is too short or the cornea is too flat.

Problem in hyperopia

The lens cannot round up enough to bend incoming light sufficiently.

Location of uncorrected hyperopia focal point

Light would focus behind the retina.

Correction for hyperopia

A convex lens provides additional convergence so light focuses on the retina.

Myopia

Nearsighted refractive error in which the distance between the cornea and retina is too long or the cornea curves too much.

Problem in myopia

Incoming light bends too much.

Location of uncorrected myopia focal point

Light focuses in front of the retina.

Correction for myopia

A concave lens diverges incoming light before it reaches the eye so the eye can focus it on the retina.

Astigmatism

Refractive error caused by irregular curvature of the lens or cornea.

Effect of astigmatism

Light rays are not evenly refracted.

Correction for astigmatism

Corrective lenses compensate for the abnormal curvatures.

LASIK

Laser-assisted in situ keratomileusis; procedure that uses a laser to remodel the cornea.

Vision problems treated with LASIK

Can be used to correct myopia and hyperopia.