Focusing the light: the cornea and lens
The cornea and lens provide focus. The cornea is a transparent dome-shaped tissue that forms the front part of your eye, gathering light and passing it through the pupil. Like a window, it is transparent and protective. Like the lens of a camera its curved surface focuses the light by bending it. The cornea provides about three fourths of the eye’s focusing power. The cornea does not have any blood vessels, so its takes its nutrients from the fluid behind it, known as the aqueous humour, as well as from the fluid in front of it, the tears, which are spread across your cornea when you blink your eyelid. The light waves passing through the eye are further focused by the two curved surfaces of the lens, which changes shape to sharpen the image’s focus.The lens is suspended between the aqueous humour and the vitreous humour, the fluid that fills the inside of the eye. The aqueous humour and the vitreous humour play an essential role in focusing the image on the retina. Because light travels at different speeds in these two fluids compared with its speed in the air, they bend any light rays that enter the eye at a non-perpendicular angle, so that these rays strike the proper place on the retina. This process is called refraction.
Light is focused by the curvature of two lenses: the fixed cornea and the flexible lens.
The curvature of the cornea also accentuates the refraction of the virtually parallel light rays that reach the eye from very distant objects. Some of these rays strike the cornea’s center. Hence they are already perpendicular to the cornea, and their angle does not change. They continue straight ahead to the centre of the retina. But other rays from these same distant objects strike the curved parts of the cornea. These rays are bent inward so that (provided the person has no vision defects) they arrive at exactly the same central point on the retina and form a focused image.
The curvature of the flexible lens is changed by the donut-shaped ciliary muscles. which form a ring around the inside of the eye. ?????
The lens is attached to the ciliary muscles by springs. When the ciliary muscles contract. it’s diameter decreases and it becomes ore round, allowing the springs to become less taut and the lens to spring back to its normal shape. , which refracts light rays more, so that they converge more. The increased refractive power that the lens thus acquires allows a crisp image of close-up objects to be formed on the retina. This phenomenon is called accommodation.
When the ciliary muscles are relaxed, the tension on the suspensory ligaments is greater, the lens the lens is flatter and refracts less, and a distant object is focused onto the retina.
Where distant objects are concerned, the cornea does most of the refracting of light rays to make them converge at a single point on the retina; the lens also contributes, but to a lesser extent. For closer objects, however (starting about 9 metres from the eye), the lens plays a far more active role in focusing the image on the retina. The light rays reaching the eye from closer objects diverge more, so they must be refracted more in order to converge on the retina. The lens alters its own shape to provide this additional refraction.
As one gets older, the lens loses its elasticity and remains too flat even when the ciliary muscles are completely contracted.
When this happens, images of near objects, such a book you are reading, are blurred.