The human eye is the sense organ of sight — a natural camera that reacts to light and pressure and can distinguish about 10 million shades of colour. These notes move from the outside inward: the protective coats, each part and its work, how light is focused into a real inverted image on the retina, the rods and cones that turn that image into nerve signals, and finally the common defects and the lenses that correct them.
The eye is a hollow ball divided into a front and a back chamber, and light has to cross several transparent structures before it can be seen.
The human eye can distinguish about how many colours?
The anterior (front) segment of the eye is made up of the cornea, lens and:
The eye reacts to light and .
The outer white shell at the back of the eye is the .
The eyeball is wrapped in three concentric layers or coats — the simplest way to begin is to peel it from the outside towards the inside.
| Layer | Also called | Composition | Function |
|---|---|---|---|
| 1. Outermost | fibrous tunic | the cornea and the sclera | protects the inner structures of the eye |
| 2. Middle | vascular tunic / uvea | choroid, ciliary body, pigmented epithelium, iris | provides blood supply and nourishment |
| 3. Innermost | — | the retina (oxygenated by choroid + retinal vessels) | contains light-sensitive cells and helps in vision |
The outermost coat of the eyeball, the fibrous tunic, is made of the sclera and:
The innermost layer of the eye, containing light-sensitive cells, is the:
The middle layer of the eye, also called the vascular tunic or uvea, is the .
The nerve leaves the eye behind the retina and carries signals to the brain.
With the coats in mind, we can name each working part in the rough order that light meets it — cornea, pupil, lens, then retina at the back.
| Part | Description / Function |
|---|---|
| Sclera | tough, thick white sheath that protects the inner parts of the eye |
| Conjunctiva | thin transparent membrane over the sclera; keeps the eye moist and clear by secreting mucus and tears |
| Cornea | transparent front layer over the pupil and iris; its job is to refract (bend) the light entering the eye |
| Iris | the coloured, pigmented part; controls the amount of light by changing the size of the pupil |
| Pupil | a small opening at the middle of the iris that allows light to come in |
| Lens | transparent structure that focuses light rays onto the retina |
| Retina | light-sensitive tissue at the back; holds millions of photoreceptors (rods and cones) that detect dim and coloured light and convert it into electrical impulses sent to the brain via the optic nerve |
| Rods | highly sensitive to dim light (night vision); e.g. an owl's eyes contain rods only |
| Cones | sensitive to bright light and colour; e.g. a hen's eyes contain cones only |
| Aqueous humour | watery fluid between the lens and the cornea; nourishes both the lens and the cornea |
| Vitreous humour | transparent jelly-like substance filling the eye; maintains the shape of the eye and refracts light before it reaches the retina |
| Optic nerve | located behind the retina; carries all nerve impulses from the photoreceptors to the brain — without it, vision is not possible |
| Macula | small central area of the retina with special light-sensitive cells; lets us see fine details clearly (centre of sharp vision) |
The coloured part of the eye that controls the amount of light is the:
Which cells are highly sensitive to dim light for night vision?
The watery fluid between the lens and cornea that nourishes both is the humour.
The small central area of the retina for seeing fine details is the .
Following a ray of light through the eye explains how we actually see; the image thrown on the retina is upside-down, and the brain turns it the right way up.
The image formed on the retina is:
Which structures together help in focusing the light?
The controls the quantity of light entering the eye.
The retina converts light into signals sent to the brain.
When the eye cannot focus an image correctly on the retina, the result is a defect of vision — most are corrected by a suitable spectacle lens.
| Defect | Description / Cause | Correction |
|---|---|---|
| 1. Myopia (near-sightedness) | distant objects look blurred, near objects clear — image is focused in front of the retina | corrected by a concave lens that focuses the image back onto the retina |
| 2. Hypermetropia (far-sightedness) | difficulty with near vision, far objects clear — image is focused behind the retina | corrected by a convex lens |
| 3. Astigmatism | the cornea/lens is irregularly shaped (warped), so the image does not focus properly | corrected by cylindrical glasses (cylindrical lens) |
| 4. Cataract | clouding of the lens prevents a clear, sharp image — results in blurred vision | (treated by surgery / lens replacement) |
| 5. Glaucoma | caused by increased intraocular pressure (IOP), usually from a fault in the eye's drainage system | (high pressure damages the optic nerve) |
Myopia (near-sightedness) is corrected by which lens?
In hypermetropia, the image is focused where?
Cataract is the of the lens, which prevents a clear image.
Glaucoma is caused by increased intraocular .
A quick gathering of the single facts examiners ask most about the eye.
The centre of sharpest vision in the eye is the:
Deficiency of which vitamin causes night blindness?
A image is formed on the retina.
The vitreous humour maintains the of the eye.
For a last look before the exam, these one-liners pull together the points asked most often.
Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.