Your eye is a converging-lens camera made of living tissue. It forms a real, inverted image on a light-sensitive screen, adjusts how much light gets in, and refocuses in a fraction of a second when you look up from your phone to the far side of the room. Every optical device in this lesson, from eyeglasses to the cameras in space telescopes, is built from the mirrors, lenses and prisms you have already studied.
| Part | Job | Camera equivalent |
|---|---|---|
| Cornea | Clear front surface; does most of the bending of light (about two-thirds) | Front element of the lens |
| Iris | Coloured muscle ring that changes the size of the pupil | Diaphragm (aperture control) |
| Pupil | The opening that lets light in; widens in dim light, narrows in bright light | Aperture |
| Lens | Flexible converging lens that fine-tunes the focus | Focusing lens |
| Retina | Layer of light-sensitive cells (rods for dim light, cones for colour) where the image forms | Image sensor |
| Optic nerve | Carries signals from the retina to the brain | Cable to the processor |
The image on your retina is real, inverted and much smaller than the object; your brain interprets it the right way up. Because the distance from lens to retina is fixed, the eye focuses on objects at different distances by changing the shape of its lens, a process called accommodation. Ciliary muscles let the lens become fatter (shorter focal length) for close objects and pull it thinner (longer focal length) for distant ones. A camera does the same job differently, by moving its lens closer to or farther from the sensor.
Vision problems and their corrections
| Condition | What is wrong | Where light focuses | Correction |
|---|---|---|---|
| Myopia (nearsightedness): near is clear, far is blurry | Eyeball too long, or cornea too strongly curved | In front of the retina | Diverging lens (negative power) spreads the rays first |
| Hyperopia (farsightedness): far is clear, near is blurry | Eyeball too short, or lens too weak | Behind the retina | Converging lens (positive power) adds extra bending |
| Presbyopia: trouble focusing up close with age, usually noticeable from the early to mid-40s | The lens stiffens and cannot accommodate | Behind the retina for near objects | Converging reading glasses, bifocals or progressive lenses |
| Astigmatism: blurry at all distances | Cornea curved unevenly, more in one direction | Not to a single point | Lenses curved differently in different directions |
Laser eye surgery reshapes the cornea itself so that glasses are no longer needed. In LASIK, a thin flap is cut in the front of the cornea and folded back, an ultraviolet laser removes microscopic amounts of tissue to change its curve, and the flap is laid back down. To fix myopia the centre of the cornea is flattened, weakening its converging power; for hyperopia it is made steeper. Contact lenses work like glasses but sit directly on the cornea.
Optical devices
- Camera: a converging lens forms a real, inverted, smaller image of a distant scene on a sensor, just like the converging-lens case with the object beyond 2F. The aperture controls the light, and the lens moves to focus.
- Microscope: an objective lens with a short focal length forms a real, inverted, enlarged image of the specimen inside the tube; the eyepiece acts as a magnifying glass on that image, giving a larger virtual image. Total magnification = objective × eyepiece, so a objective with a eyepiece gives .
- Refracting telescope: a large objective lens with a long focal length collects light from a distant object and forms a real image near its focal point; the eyepiece magnifies that image.
- Reflecting telescope: a large concave mirror replaces the objective lens. Big mirrors can be supported from behind and do not split colours the way lenses do, so all of the world's largest telescopes are reflectors.
- Binoculars: a pair of short refracting telescopes. Inside, glass prisms use total internal reflection to flip the inverted image upright and fold the long light path into a compact case.
- Optometrist: examines eyes, tests vision and prescribes glasses and contact lenses (a university Doctor of Optometry degree; Ontario's program is at the University of Waterloo).
- Ophthalmologist: a medical doctor who specializes in eye disease and eye surgery, including laser surgery.
- Optician: fits and dispenses eyeglasses and contact lenses from a prescription.
- Optical engineer: designs lenses, cameras, laser systems and fibre-optic networks.
- Photographer: chooses lenses, apertures and focus to control exactly how light reaches the sensor.