3.18 - The Eye & Vision
The structures of the eye
The eye focuses light onto the light-sensitive retina at the back of the eye, where an image forms.
Parts of the eye
- Cornea - The transparent, curved layer at the front of the eye. It does most of the focusing.
- Iris and pupil - The iris is a coloured ring of muscle that changes the size of the pupil (the hole in the middle) to control how much light enters.
- Lens - A converging lens behind the pupil that changes shape to fine-tune focusing.
- Ciliary muscles - A ring of muscle that changes the shape of the lens.
- Suspensory ligaments - Fibres connecting the ciliary muscles to the lens.
- Retina - The light-sensitive layer at the back of the eye where the image forms. Nerve cells send signals from the retina to the brain.
How the eye changes focus
- Near object - The ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter and more powerful.
- Distant object - The ciliary muscles relax, the suspensory ligaments pull tight, and the lens becomes thinner and less powerful.
Near point, far point and range of vision
- Near point - The closest distance an object can be and still be focused on sharply (about 25 cm for a normal eye).
- Far point - The furthest distance an object can be and still be focused on sharply (infinity for a normal eye).
- Range of vision - The distance between the near point and the far point.
The eye and the camera compared
Both use a converging lens to form a real, inverted image on a light-sensitive surface.
What matches
| In the eye | In the camera |
|---|---|
| Cornea and lens | Lens |
| Iris and pupil | Aperture |
| Retina | Film, or a CCD sensor |
How they focus
- Eye - Keeps the distance between the lens and retina fixed and changes the shape of the lens (fatter for near objects, thinner for distant ones).
- Camera - Keeps the shape of the lens fixed and changes the distance between the lens and the film or sensor.
Lasers in surgery
A laser produces a narrow, highly concentrated beam of light that delivers a large amount of energy to a very small area.
Surgical uses
- Cutting - The beam heats tissue until it vaporises, making a very fine cut.
- Cauterising - The heat seals blood vessels as it cuts, reducing bleeding.
- Burning - Destroys unwanted tissue without cutting into surrounding areas.
Laser eye surgery
- A laser corrects short sight and long sight by removing a thin layer of the cornea to change its curvature.
- Changing the cornea's shape changes how strongly it refracts light, focusing light directly onto the retina.
TermDefinition
RetinaThe light-sensitive layer at the back of the eye where the image forms.
CorneaThe transparent, curved layer at the front of the eye that does most of the focusing.
IrisA coloured ring of muscle that changes the size of the pupil to control how much light enters.
Near pointThe closest distance an object can be and still be focused on sharply.
Far pointThe furthest distance an object can be and still be focused on sharply.