How the eyes work
The eyes are two little miracles we carry around with us, mostly taking them for granted. They are complex in their details but how they work is basically simple. They work like a traditional camera.
Using a viewfinder called the cornea, the eyes (and brain) choose something to look at. The cornea has several functions and one is to act as a lens, bending light as it initially enters the eye.
The light carries images from the view we are looking at, and after coming through the cornea, it travels through an aperture called the pupil. The human iris automatically sets the aperture size according to the intensity of light at any given time. Let’s say the view is of a tree up ahead on a road we’re driving along. The light then travels through a lens. It is refracted (focused) by the lens and continues traveling inward.
It reaches the film, called the retina, at the back of the eye. The retina has special light-sensitive cells that register images and convert them to electrical energy. Near its centre is the macula, which gives our central vision because it has the most cones – cells that perceive detail and colour in bright light. Around the retinal periphery are mostly rods – cells that provide night vision.
Like a messenger service, the optic nerve collects all these images and carries them to the dark room – the brain’s vision centre. The brain interprets the electrical energy as images, and now we know that we are on a road with a tree up ahead.
All of that happens in a flash. If you glance from the tree down at your watch, the eye’s lens automatically changes its shape. It becomes more sharply curved so as to focus the light coming from your watch, which is only about a foot away. If you check your watch against the clock on the dashboard, the lens flattens somewhat. When you glance back along the road, the lens becomes flatter again. This lens’ ability to change its shape is called Accommodation and in this way, a person under 40 years of age with 20/20 vision can see clearly at all distances.
As we all know, not everybody is born with 20/20 vision. Many are shortsighted (myopic), many have astigmatism, and some people are longsighted (hyperopic).
In a myopic eye, the cornea is too steeply curved or the eyeball length is too large and light from far away objects focuses too soon, before it reaches the retina. This creates blurry distance vision.
In a hyperopic eye, the cornea is too flat or the eyeball length is too small. Light from close-up objects is not focused soon enough, and it has not yet focused by the time it reaches the retina. This creates blurry near vision.
In an astigmatic eye, the cornea has an oval shape instead of spherical. This creates two focal points – think of a rugby ball, with a flatter curvature on its long side and a steeper curvature on its short side. This creates blurriness at all distances.
The Eye’s Fluids
The eyes are filled with fluids – all transparent so as to allow light through to the retina. Tears keep the cornea and white part (the sclera with overlying conjunctiva) moist and comfortable. Aqueous fluid bathes the lens and the back of the cornea. Vitreous fluid bathes the retina and helps to keep the retinal cells healthy. The internal (aqueous and vitreous) fluids exert outward pressure on the eyeball wall that helps the eyeball to keep its spherical shape.