Foundations of ophthalmic science

Reflection

When light reflects off a surface at the same angle it arrived. The angle of incidence equals to the angle of reflection. Make the ophthalmoscope work.

Refraction

The bending of light when it crosses from one medium to another. Bending and directional changes depend on both materials. This focus images into the retina.

The Snell’s Law and Refraction Index

Give the exact amount of measurement between the incident angle, refracted angle, and the media’s refractive indices.

Refractive index- measures how much a material slows light relative to vacuum.

A higher index slows and bends the light.

Light bends toward the normal when entering a denser medium and bends away when exiting.

Snell’s Law formula

n1x sin(01)=n²x sin(02)

Light passes through five different media

Air- Refractive index

Tear film cornea

Aqueous Humor

Crystalline lends

Vitreous

Air- Refractive Index

1.000. The starting point before entering the eye

the focusing power is about forty-three diopters. The lens adds fifteen to twenty diopters and changes shape to focus. This is why corneal disease or cataracts disrupt vision.

Tear film cornea

n=1.376. Major refractive power of 43 diopters

A thin, vital, moist coating that covers the cornea; it consists of three layers: The lipid layer, the aqueous layer, and the Mucas layer.

Functions

Vision Refraction: Smooths out microscopic irregularities on the cornea so light focuses properly

Protection: Shields the eye from dust, debris, and foreign particles

Defense: Fights off bacteria and microbes using special proteins and antibodies

Lubrication: Reduces friction between the eyelid and the cornea during blinking.

Aqueous Humor

n=1.336. Transparent fluid filling the anterior chamber (front) of the eye.

maintain intraocular pressure (IOP) to keep the eyeball spherical, nourish avascular structures like the lens and cornea, remove metabolic waste, and refract light.

Vitreous Humor

n=1.386. Gel-like substance filling the space between the lens and the retina.

Maintain eye shape- the gel-like filling creates internal pressure, keeping the eyeball in shape

Retinal Support: It gently presses the retina against the back of the eye, preventing it from detaching or shifting

Vision Clarity: Being 98% to 99% water and transparent, it acts as a clear medium that allows light to travel unobstructed to the retina for sharp vision

Shock absorption: It cushions the eye against sudden head movements and physical impacts

Oxygen balance: helps maintain an optimal oxygen gradient within the eye, ensuring a lower concentration near the lens and a higher concentration near the retina

Metabolic transport: contains proteins, sugars,s and salts that supply vital nutrients to avascular ( blood vessel-free) tissue in the eye.

Crystalline lens

n=1.386-1.406. Has 15-20 diopter, it is a clear biconvex structure inside the eye located behind the iris and pupil.

Refracts and focuses light onto the retina, adjust focus for different distances through accommodation, and blocks harmful ultraviolet rays

Vergence

How light converges, diverges, or stay parallel. Measured in diopters, it is the language of prescription. The parallel rays from a distant object have zero vergence. Converging rays have positive vergence and diverging rays with negative vergence.

Why diopters are important

When you write a plus or minus diopter, you’re describing vergence, every lens adds or subtracts vergence to redirect light into the retina.

Diopter

unit of measurement used to measure the optical power of a lens or curved mirror. (D=1/f)

focal point

where parallel light rays converge after lens or appear to diverge

converging lens

parallel rays converge to a real focal point. Has a positive diopter. In a hyperopic eye( farsighted), the focal point sits in front of the retina

Diverging lens

spread the rays creating a virtual focal point on the same side. Focal length is the distance from lens to that point. Short focal length represents high power.

Refractive Errors

Occur from a mismatch between the eye bending power and its length

Myopia (nearsighted)

The eye bends too much, so the focal point falls in front of the retina. A minus lens reduces that bending before light enters the eye, pushing the focal point back.

Symptoms: eye strain, headaches, and sitting too close to the TV or classroom board.

Hyperopia (nearsighted)

A refractive error where light entering the eye focuses behind the retina instead of directly on it. Makes distant objects appear clearer than nearby items, and often causes eye strain, headaches, and difficulty with reading tasks.

aberration

An optical imperfection where light rays fail to focus perfectly on the retina, causing visual distortions like blurry vision, glare, or halos.

Two classes: lower-order and High-order

Lower-order aberration

defocus and distorted vision. Accounts for 85% of optical error and it’s corrected with standard lenses.

High-power aberrations

Three parts: coma, spherical, and trefoil

Not correctable with standard lenses because standard lenses only fix lower power optical imprefection. Measured by wavefront aberrometry. Clinically appear after LASIK, Keratoconus, or under dim light.

Keratoconus

An eye condition where the clear dome-shaped cornea thins and bulges outward into a cone shape.

LASIK

Outpatient refractive surgery that permanently reshapes the cornea suing excimer laser to correct vision issues such as nearsightedness, farsightedness, and astigmatism.

Spherical aberraion

Peripheral ( edge) zones of a curved surface. Bend light more than the central zone. In the eye, rays through the pupil edge focus slightly in front of the central rays.

The pupil limits this by constricting in bright light, making vision sharper during daylight. Aspheric intraocular lenses (IOLs) are designed to counteract this constriction after cataract surgery.

Coma

Higher-order aberration that affects off-axis light. Off-axis rays focus at different height creating a comet-shaped blur. Instead of a sharp point, the patient sees a streak or tail.

Common after decentered corneal refractive surgery and keratoconus. Patients often see halos, ghosting, or flare at night even with a normal prescription.

wavefront aberrometers

measure all the eye’s optical errors not just sphere and cylinder. A grid of light enters the eye; the device analyzes how the reflected wavefront is distorted. the output is a color-coded map of every aberration present. A proper alignment adds equate pupil dilation when required and stable fixation is essential.

reflection instruments

Not just a physics concept; it’s built into your daily instrument. Corneal topographers: map curvature using Placido ring reflections off the tear film alignment

The red reflex in an ophthalmoscope is reflected from the fundus.

Fundus-inner back surface of the eyeball; it includes the retina, optic disc( Where the optic nerve enters), macula, fovea, and retinal blood vessels.

Slit-lamp alignment uses specular reflection from the cornea.

Linking principles to instrument

the autorefractor uses vergence analysis to measure refractive error. The corneal topographer maps corneal shape by reflecting light off the rear film. the wavefront aberrometer captures the eye’s optical fingerprint.

autorefractor

vergence and focal point analysis, measure refractive error in sphere, cylinder, and axis. Need a stable fixation and proper alignment

corneal topographer

Reflection off the tear film surface: measures corneal curvature and shape. The accuracy is affected by dry eye( always check the tear film).

Corneal Curvature- Eye transparent outer surface, providing about two-thirds of the eye's focusing power.

wavefront aberometer

Full-wavefront analysis that measures lower- and higher-order aberrations. Consider the pupil size and dilation when using this instrument

Three layers of the tear film cornea

Lipid (oily layer): The outermost layer, produced by the meibomian glands in the eyelids; it prevents quick evaporation.

Aqueous (watery) layer: The middle layer. Produced by the lacrimal glands; it supplies moisture, oxygen, and nutrients to the cornea.

Mucus (Mucin) layer: The innermost layer. Produced by goblet cells; it helps the watery layer stick evenly to the front of the cornea