Basics
Depending on the chemical structure of all surfaces (all the unique materials said surfaces can be made of), light is reflected in its own unique way. Our eyes, on the other hand, are constantly picking up any changes in the reflected light, coming from the objects we focus our attention on.
Our brain can then processes the information collected by the eyes and it all culminates with a coherent image of our surroundings. Or what we call, to see.
There are billions and billions of light rays flying all around us. Scattered by the air or reflected from the ground and various objects around you. They are what led our ancient ancestors to evolve into having a visual organ that can understand the information these rays were carrying.
But how do they extract this information exactly? Imagine a field, a field of 100-million light-sensitive sensors that together form a dense labyrinth made out of nerve cells where each single one is a specialist in tracking down parts of the electromagnetic radiation.
What you imagined is probably an abstract version of the thin layer of nerve tissue we call, the retina. It is hidden from all the noise at the back of the eye. Almost in complete darkness and constantly waiting.
When our eyes are open, light is projected onto the retina through the optics of the eye: the cornea, the iris and the lens.
Optics of the eye
The design of the human eye is far from what its idealistic reputation suggests, as if created from a higher being. It is a rather pragmatic chain of small adjustments to all its previous versions. A complex outcome of gradual improvements spread throughout the many years of human evolution.
Light enters the eye through an outer protective layer called the cornea. Since life developed in water, the original purpose of the cornea was to simply isolate the lens from the surroundings.
However, when we stepped out of deep waters, the cornea transformed into having a more complex, spherical form. Its purpose also shifted and it began to focus the image before it entered the lens, independently of the difference in density between water and air.
Iris
A bit deeper, behind the cornea, is the very sensitive to any changes in the intensity of light, iris. In fact, this flexible organic structure plays the same role as the diaphragm found in modern day cameras.
It instantaneously adjusts to the amount of light entering the eye and impacts the exposition of the image produced in our brain.
You can also easily understand what the iris does by observing the dynamic shrinking and expanding of the pupil - the hole at the center of the iris through which it can control how much light can reach the eye lens.
Lens
The lens in our eye is almost entirely made out of protein fibers. This is a remarkable fact when we keep in mind its 80% transparency ( a window glass is around 90%). At the same time, it is actually because of this high concentration in protein (approximately 300 mg), that can the tissue have its refractile properties .
The most crucial quality of the lens is its flexibility. By default, the fibers are stretched by the suspensory ligaments of the lens to allow for a distant point of focus. When the ciliary muscles surrounding the lens contract, the refractive power increases, thus allowing us to focus on nearby objects.
Since the lens is quite literally a biconvex lens, it transmits the light rays from the point of their origin straight to the iris. This results in a reversed virtual image projection and although our eyes see the world upside down, this raw image we receive is inverted back right-side up by our brain.
Image processing
We can’t observe the world around us without our eyes, but what we call “to see” comes from our brain. When our eyes catch the oncoming light, they transfer this information to our brain via an optic nerve. There are in fact two optic nerves, one for each eye, and they cross at a certain place called the optic chiasm.
There, the information from the left side of both eyes and right side of both eyes are combined to form one accurate image of our surroundings.
After the visual data is processed, it is sent to the visual cortex, located at the back of our brain, for the image to be reconstructed and “seen” by us. Of course, all of the above happens too fast for us to even have a chance to notice.
Our ability to see is a process that involves many external factors and just as many internal factors for it to become possible. If observed on a more microscopic level, it is a very complex sequence of events between whole micro-worlds, each with its own unique task. The human eyes are a biological marvel that played a key role in the progression of our species and continue to do so.
