Function of the visual organs
The eye can be compared to a camera: the cornea and lens refract light, the pupil acts as a diaphragm controlling light, and the retina is the screen receiving the image. Rays from an object form a small inverted image on the retina, impulses go through the optic nerve to the occipital cortex, and the brain perceives the image upright and whole. Accommodation is the change in lens curvature: when looking near, ciliary muscles contract and the lens becomes more convex; for far it flattens. With two eyes we see an object as one and in three dimensions. In myopia the image forms in front of the retina (the eyeball is longer), in hyperopia behind it (the eyeball is shorter); glasses or lenses prescribed by a doctor correct this. Testing vision and choosing glasses is solely an eye doctor’s job. Eye hygiene: read in good light, hold a book about 30–40 cm from the eyes, do not read in a moving vehicle, every 20 minutes look into the distance for 20 seconds, limit screen time, wear good sunglasses on sunny days, and eat foods rich in vitamin A (carrot, liver, eggs, greens). If the eyes turn red, water a lot or vision blurs, tell adults and see a doctor.
The 20–20–20 rule: after 20 minutes of screen or book, look 20 seconds at something 6 metres (20 feet) away. Mark how many times you did it in a day and note how your eyes felt.