Physics of Lenses and Vision

The Eye: Optics of Vision

• Understanding the eye's optical system requires knowledge of light refraction, focusing, and depth of focus.
• Refractive index of a substance is the ratio of light's velocity in air to its velocity in that substance. Air's refractive index is 1.00.
• Light travels slower through transparent mediums than air, affecting its velocity and wavelength.
• Refraction is the bending of light at an interface between two mediums with different refractive indices. The degree of bending depends on the refractive index ratio and angle between the interface and incident light.
• Convex lenses converge parallel light rays to a focal point. Light passing through the center of the lens doesn't refract.
• Concave lenses diverge parallel light rays, the rays spreading out. Light through the center of the lens doesn't refract.
• Cylindrical lenses refract light in only one plane, focusing light to a line instead of a point.
• Focal length of a convex lens is the distance beyond it where parallel rays converge to a common point.
• A convex lens focuses diverging light rays further from the lens than its focal length for parallel rays.
• The eye's lens system has four refractive interfaces: air/cornea, cornea/aqueous humor, aqueous humor/lens, and lens/vitreous humor.
• The eye's refractive power is measured in diopters.
• One diopter is equal to 1 meter divided by the focal length of a convex lens.
• Concave lenses are measured by their ability to diverge light; a strength of -1 diverges light as a +1 convex lens converges it.
• Concave lenses neutralize convex lenses.
• Parallel to the other lenses, the eye's normal refractive power is 59 diopters.
• The refractive power of the cornea (air/cornea interface) accounts for most of the eye's refractive power.

Accommodation

• Accommodation is the eye's ability to change the shape of its lens to focus on objects at various distances.
• Children have a greater range of accommodation, from 20-34 diopters.
• The ligaments attached to the lens maintain its form via tension; the ciliary muscle adjusts this by relaxing or contracting the ligaments.
• Contraction of meridional and circular fibers in the ciliary muscle relaxes the suspensory ligaments allowing the lens to become more rounded, increasing its refractive power.
• This process uses the sympathetic nervous system fibers and parasympathetic nerve signaling from the brain.
• Presbyopia is the loss of accommodation as the lens loses elasticity with age, reducing its ability to change shape. Accommodation reduces to virtually zero in old age.

Errors of Refraction

• Emmetropia (normal vision): Parallel light rays focus correctly on the retina with the ciliary muscle relaxed.
• Hyperopia (farsightedness): Parallel light rays focus behind the retina with the ciliary muscle relaxed. Accommodation helps focus near-objects.
• Myopia (nearsightedness): Parallel light rays focus in front of the retina with the ciliary muscle relaxed. Distant objects are blurry. Accommodation doesn't help focus distant objects.

Depth Perception

• Depth perception is the ability to perceive 3D space.
• Three main means of depth perception:
• size of retinal images of known objects
• parallax (relative movement of images on the retina when head/eyes move, useful for close objects)
• stereopsis (binocular vision; images of nearby objects on each retina are slightly different, better for close distances)

Fluid System of the Eye

• Aqueous humor is the fluid in front of the lens.
• Vitreous humor is the fluid behind the lens.
• The fluid fills and maintains pressure in the eye. Maintain constant pressure and resist flow, especially for outflow.
• Aqueous humor formation occurs in the ciliary body; flow out occurs through the trabecular meshwork, aqueous veins, and the canal of Schlemm.
• Intraocular pressure (IOP) is measured using a tonometer; normal range is 12-20 mm Hg.
• High IOP can damage the optic nerve and lead to glaucoma.

Visual Acuity

• Visual acuity describes the sharpness of vision.
• Optimal vision allows for the distinction of two or more light points.
• Visual acuity is tested by presenting letters of varying sizes at a fixed distance.
• Normal visual acuity is 20/20; 20/200 vision means the person sees at 20 feet what a person with normal vision sees at 200 feet.
• Visual acuity decreases with age due to many factors, including lens thickness changing, curvature of lens, and presbyopia.