Structure and function of the eye

Medical Physics | AQA A-Level Physics

The corneaThe transparent front surface of the eye. It does most of the refraction (about two-thirds of the eye's total power) because of the large refractive index change from air to cornea. is the main refracting surface of the eye. It provides about two-thirds of the total refractive power because of the large change in refractive index from air ($n = 1.00$) to cornea ($n = 1.38$).
The irisThe coloured ring of muscle that controls the size of the pupil, regulating the amount of light entering the eye. controls the size of the pupilThe opening in the centre of the iris through which light enters the eye., regulating how much light enters the eye.
The lensA transparent, flexible structure behind the iris that fine-tunes the focus by changing shape. It provides the remaining one-third of the eye's refractive power. fine-tunes the focus. It can change shape (become fatter or thinner) to adjust its focal length. This process is called accommodationThe process by which the ciliary muscles change the shape of the lens to focus on objects at different distances. The lens becomes fatter for near objects and thinner for distant objects..
The ciliary musclesRing-shaped muscles attached to the lens by suspensory ligaments. When they contract, the lens becomes fatter (shorter focal length) for near vision. When they relax, the lens is pulled thin (longer focal length) for distance vision. contract to make the lens fatter (more powerful) for near objects, and relax to let the suspensory ligaments pull it thin for distant objects.
The retinaThe light-sensitive layer at the back of the eye containing rods and cones. It acts as the screen on which the image is formed. contains light-sensitive cells (rods and cones) and acts as the screen where the image is formed.
The foveaA small pit in the centre of the retina packed with cones. It provides the sharpest vision and best colour discrimination. is a small region at the centre of the retina densely packed with cones. It gives the sharpest vision.
The optic nerveThe nerve that carries electrical signals from the retina to the brain for processing. carries signals from the retina to the brain. Where it connects, there are no rods or cones, creating the blind spotThe point on the retina where the optic nerve exits the eye. There are no photoreceptors here, so no image is detected at this point..
The near pointThe closest distance at which the eye can focus on an object clearly. For a normal eye, this is about 25 cm. is the closest distance the eye can focus (about 25 cm for a normal eye). The far pointThe farthest distance at which the eye can focus clearly. For a normal eye, this is infinity. is the farthest (infinity for a normal eye).