The Eye and Technologies for Seeing Better

The camera you were born with

Your eye is an optical instrument that focuses light, controls brightness, and sends images to your brain thousands of times a minute. When it doesn't focus perfectly, technology can help: eyeglasses, contact lenses and laser surgery. Other technologies extend human vision into light we can't see at all. In this lesson you will use what you know about lenses to explain how the eye works and to evaluate these technologies.

Parts of the eye

• Cornea: the clear, curved front surface. It does most (about two-thirds) of the eye's focusing, because the biggest change in index of refraction is from air to cornea.
• Iris: the coloured ring of muscle that changes the size of the pupil, the opening that lets light in. In bright light the pupil shrinks; in dim light it widens.
• Lens: a flexible converging lens that does the fine focusing. The ciliary muscles change its shape: rounder (shorter focal length) for near objects, flatter for distant objects. This adjusting is called accommodation.
• Retina: the light-sensitive layer at the back. Rods work in dim light (black and white); cones detect colour and fine detail.
• Optic nerve: carries signals to the brain. Where it leaves the eye there are no rods or cones: this is the blind spot.

The cornea and lens together act as a converging lens. The image on the retina is real, inverted and smaller, just like in a camera. Your brain flips it so you see the world right side up.

Common vision problems

• Nearsightedness (myopia): distant objects are blurry. The eyeball is a little too long (or the cornea too curved), so light from distant objects focuses in front of the retina. Correction: a diverging lens spreads the rays out slightly so the eye focuses them farther back, on the retina.
• Farsightedness (hyperopia): near objects are blurry. The eyeball is too short (or the focusing is too weak), so light would focus behind the retina. Correction: a converging lens adds extra bending.
• Presbyopia: with age (often starting in a person's 40s), the lens becomes stiffer and cannot become round enough to focus on near objects. Correction: converging reading glasses, bifocals or progressive lenses.
• Astigmatism: the cornea is curved more in one direction than another (shaped a bit like the back of a spoon instead of a ball), so vision is blurry or stretched at all distances. Correction: lenses curved more in one direction (cylindrical lenses).

Lens power: reading a prescription

Eye-care professionals describe lenses by their power (P) instead of their focal length:

P = 1 / f, with f in metres. The unit is the dioptre (D).

• A converging lens has positive power; a diverging lens has negative power.
• A stronger lens (shorter focal length) has a larger power.

Evaluating vision technologies

To evaluate a technology, judge it against clear criteria: how well it works, safety and risks, cost, convenience, and who can use it.

• Eyeglasses: simple, safe and relatively inexpensive; easy to update when a prescription changes. But they can fog up in winter when you come in from the cold, slide down during sports, and limit side (peripheral) vision at the edges of the frames.
• Contact lenses: thin lenses that sit on the tear film over the cornea; they move with the eye, giving a wider field of view, and are good for sports. But they require careful hygiene; misuse (sleeping or swimming in lenses not meant for it, poor cleaning) can cause serious eye infections.
• Laser eye surgery (such as LASIK and PRK): a precise ultraviolet laser reshapes the cornea, changing its focusing power so the image lands on the retina. Many patients no longer need glasses for distance vision. Risks include dry eyes and glare or halos around lights at night, and rarely more serious problems. It is not suitable for everyone (for example, people whose prescription is still changing, as it often does in the teen years), and in Ontario it is generally paid for by the patient, not by OHIP. Some clinics use very fast femtosecond lasers, which rely on chirped pulse amplification, the technique that earned Canadian physicist Donna Strickland (University of Waterloo) a share of the 2018 Nobel Prize in Physics.
• Seeing light we can't see: thermal (infrared) cameras detect the infrared radiation that warm objects give off. Firefighters use them to find people in smoke-filled rooms, search-and-rescue teams use them at night, and energy auditors use them to find where a house is losing heat in winter. Image-intensifier night-vision devices amplify the tiny amount of light available at night.
• Inuit snow goggles: for many generations, Inuit have carved snow goggles from materials such as wood, bone or antler, with narrow slits to look through. They cut down the intense light reflected from snow and ice and help prevent snow blindness, a painful sunburn of the cornea caused by ultraviolet light. Inuit makers continue to carve them today, and modern sunglasses and ski goggles with UV protection do the same job.

Careers in vision and optics

• Optometrist: examines eyes, diagnoses vision problems and prescribes glasses and contact lenses. Requires several years of university study, including a Doctor of Optometry (OD) program (four years at the University of Waterloo, after at least three years of university science). In Canada, optometry is taught at the University of Waterloo (in English) and the Université de Montréal (in French).
• Ophthalmologist: a medical doctor who specializes in eye diseases and eye surgery, including laser surgery. Requires medical school and several more years of specialized residency training.
• Optician: fits and dispenses eyeglasses and contact lenses from a prescription. In Ontario, opticians complete a college program and are regulated by a professional college.
• Others: vision scientists, photonics and laser technicians, optical engineers who design cameras and sensors.