The eye and vision
The outer coat of the eyeball is the sclera, and its transparent front part is the cornea. The hole in the centre of the iris, the pupil, controls the amount of light. The eye lens is a biconvex lens (n ≈ 1.4–1.5): it forms a real, inverted, reduced image of the object on the retina, nerve fibres carry it to the brain, and the brain perceives it upright. Muscles change the curvature of the lens and so its focal length (accommodation), which is why far and near objects are sharp on the retina. In short-sightedness the image forms in front of the retina (the optical power is greater than normal), which is corrected by spectacles with a concave (diverging, D < 0) lens. In long-sightedness the image forms behind the retina, corrected by a convex (converging, D > 0) lens. Seeing with two eyes helps judge the distance to an object and spatial depth. Spectacles are chosen after an examination by a doctor.
Observation (safe): in a bright room watch your pupil in a mirror, then darken the room (close the curtains) and see how the pupil size changes. Never aim a strong light at the eyes. Eye hygiene: reading distance 30–40 cm, enough light, breaks from screens.