Light, refraction and focus
Light travels in straight lines through a uniform medium, but bends when it passes from one transparent substance into another in which it travels at a different speed. This bending is called refraction. Snell’s law describes it: the amount of bending depends on the angle at which light meets the boundary and on the refractive indices of the two materials. A curved boundary can bring parallel rays together to a point, which is how a lens forms an image.
The eye relies entirely on this principle. Its job is to bend light from each point in a scene so it converges to a matching point on the retina, giving a sharp picture.
Two focusing elements
The eye has two main refracting parts:
- The cornea. The front surface of the eye is strongly curved, and the jump in refractive index between air and the corneal tissue is large. Because of this, the cornea does most of the eye’s total bending of light, commonly cited as about two thirds of it or more.
- The lens. Inside the eye the lens refines the focus. Its refractive power is smaller than the cornea’s, because it is surrounded by fluids with a similar refractive index, but it can change.
Optical power is measured in diopters, equal to one divided by the focal length in meters. The whole relaxed eye is often cited at roughly 60 diopters, which for a lens in air would correspond to a focal length of about 17 millimeters (1 ÷ 60 ≈ 0.017 m). The actual eye is filled with fluid and so focuses at a somewhat longer distance, matching its length of roughly 24 millimeters from front to back. These are typical figures and vary between people.
Accommodation: refocusing on the fly
For a lens of fixed power, objects at different distances come into focus at different distances behind the lens, as the thin lens relation shows: 1/f = 1/object distance + 1/image distance. Since the retina sits at a fixed distance, the eye must change its focal length to see near and far objects clearly. It does this by changing the shape of the lens, a process called accommodation. A ring of ciliary muscle around the lens tightens, letting the lens bulge and become more curved for near viewing, and relaxes for distance viewing.
The range of accommodation is only a few diopters in adulthood and shrinks with age as the lens stiffens, which is why many people need reading glasses in their forties or later. This stiffening is called presbyopia.
The pupil as an aperture
The iris sets the size of the pupil, which acts as an aperture. Its diameter typically ranges from roughly 2 millimeters in bright light to about 8 millimeters in darkness, though the range varies and narrows with age. A larger pupil admits more light, but it also exposes more of the lens and cornea to imperfections. A smaller pupil reduces certain blurring errors and increases depth of field, the range of distances that appear acceptably sharp. Making the opening very small brings its own limit, because of diffraction, the spreading of light waves passing through a small hole.
Image formation on the retina
The optics produce a real, inverted image on the retina, flipped top to bottom and left to right. The brain’s interpretation takes this into account, so the world does not seem upside down. The retina samples the image with its light-sensitive cells and converts it to electrical signals, a biological stage discussed in the eye’s anatomy.
The image is sharpest at the center of the visual field. Away from the center, detail falls off considerably because the cells there are spaced more widely. The eye compensates by moving rapidly to put interesting details onto the sharpest region.
When focus goes wrong
Many common vision problems are simply geometry.
- Myopia (nearsightedness): Light from distant objects focuses in front of the retina, often because the eyeball is slightly too long or the optics are too strong. A diverging (concave) lens in glasses or contacts spreads the rays a little and moves the focus back.
- Hyperopia (farsightedness): Focus would fall behind the retina, because the eye is short or has low power. A converging (convex) lens can correct it, and young eyes can often compensate by accommodating.
- Astigmatism: The cornea or lens is not equally curved in every direction, so light focuses at different distances in different directions. Cylindrical lenses correct it.
- Presbyopia: Loss of accommodation, as above, usually corrected for near work with convex lenses.
Eyes also have optical imperfections called aberrations, which blur images slightly. The brain and retina adapt to these, so most are not noticed.
Why the same physics appears in tools
The principles at work in the eye are the same ones used in glasses, cameras, microscopes and optical telescopes. Camera designers copy ideas such as the adjustable aperture, and researchers building computer vision systems often compare artificial image capture with the way the eye samples a scene. Light itself is a small part of the electromagnetic range, known as the visible spectrum.
This article is a general physics explanation and not an eye-care recommendation. For questions about vision, see a qualified eye-care professional.