The eye Image formation Accommodation Focusing on near objects Focusing on distant objects Eye defects: short sight, long sight, colour blindness

The eyes one of the 5 sense organs detect light and form images provide the sense of sight each eye consists of an eyeball located in the orbits protected by the skull held by 3 pairs of eye muscles

Image formation Light from an object is refracted by vitreous humour aqueous humour cornea lens

Image formation Light focused onto retina to form a real, inverted and smaller image than the object.

Rods and cones detect the image and cause nerve impulses. Image formation

Impulses are transmitted along optic nerve to the optic centre in the brain.

The brain interprets the impulses and sees an upright image of the object. Image formation

Accommodation The ability of the eye to focus objects at different distances onto the retina By changing curvature of the lens –Near objects  more convex (thicker) –Distant objects  less convex (thinner)

Focusing on near objects Circular ciliary muscles contract. Tension in the suspensory ligaments reduced. Lens becomes more convex under its own elasticity.

Focusing on near objects Light is refracted more by the lens. Eyes become tired if focused on near objects for too long. Light is focused on the yellow spot.

Focusing on distant objects Circular ciliary muscles relax. Tension in suspensory ligaments increases. Lens is pulled to become less convex.

Focusing on distant objects The eye is at rest. Light is refracted a little by the lens. Light is focused on the yellow spot.

Eye defects: short sight Can see near objects but not distant objects Image of a distant object is formed in front of the retina Cause(s): lens is too convex or eyeball is too long Correction: wear concave lens to diverge light rays

Eye defects: long sight Can see distant objects but not near objects Image of a near object is formed behind the retina Cause(s): lens is too thin or eyeball is too short Correction: wear convex lens to converge the light rays

Eye defects: colour blindness Inability to see some or all colours due to reduced number of or some defects in one or more of the three types of cone cells Most common form: red-green colour blindness due to lack of red or green cone cells Can be inherited

Summary The eyes give the sensation of sight Image formation Light is refracted by cornea, aqueous humour, lens and vitreous humour Image is formed on the retina, detected by the sensory cells (rods and cones) Nerve impulses are transmitted along optic nerve and interpreted by the brain

Accommodation near distant more convex less convex ObjectLens By changing the curvature of the lens

Eye defects short sight lens too convex eyeball too long wear concave lens long sight lens too concave eyeball too short wear convex lens colour blindness defect in cone cells — DefectCause(s)Correction