Everything about the human body is marvellous, but when it comes to human vision, this plays the central, most fundamental role in enhancing what we call the “human experience” here on earth. We see, therefore we are ! All of the magnificent beauty of the Physical world owes its appreciation to a few millimeters wide Cornea, through which Light rays enter the eye, the clear front “window” of the eye. Through this 11.5 mm wide cornea we gaze at the Cosmos, the vastness of which is still a mystery. The structure of the human eye is so complex that it’s hard to believe that there are people who still deny the existence and the overwhelming power of the Intelligent Designer, Creator and Sustainer.<h1>How do we see?</h1>
The Cornea is the clear, transparent front covering which admits light and begins the refractive process. It also keeps foreign particles from entering the eye. It bends the light rays in such a way that they pass freely through the pupil. We can tolerate very large scars on our bodies with no concern except for our vanity. This is not so in the cornea. Even a minor scar or irregularity in the shape can impair vision. No matter how well the rest of the eye is functioning, if the cornea is scarred, clouded or distorted, vision will be affected. The cornea is so masterfully engineered that even the most expensive man made lenses cannot match its precision.
The iris works like a shutter in a camera. It has the ability to enlarge and shrink, depending on how much light is entering the eye.
After passing through the iris, the light rays pass through the eye’s natural crystalline lens. This clear, flexible structure works like the lens in a camera, shortening and lengthening its width in order to focus light rays properly. All of these movements of the lens and iris happen so instantaneously within a fraction of a second. This camera as you can see, does not hang!
Light rays pass through a dense, transparent gel-like substance, called the vitreous that fills the globe of the eyeball and helps the eye hold its spherical shape.
The retina is covered in millions of light-sensitive receptors known as rods and cones. When light reaches the rod and cone cells a series of complex chemical reactions take place, forming the chemical Rhodopsin, that converts the light into electrical impulses. These electrical impulses through millions of tiny nerve endings, from the optic nerve to the visual cortex of the brain. First, they move through interneurons and then to neurons known as ganglion cells. These cells are cross-linked, able to compare adjacent signals, filtering out some of the information before passing it on to the brain. This helps to improve contrast and definition. As the two optic nerves enter the brain, they cross over, coming together at a point known as the optic chiasm. Here, signals from the left side of both eyes are diverted to the left side of the brain, and vice versa, allowing the images from both eyes to be combined and compared.
The signals enter the brain via the thalamus, which separates the incoming information into two parts, one containing colour and detail, and the other movement and contrast. The messages then move to the back of the brain, and into the visual cortex. The cortex is laid out so that it mirrors the back of the retina, allowing a detailed image to be reconstructed.<h1>Do we see from our eyes or our brain ?</h1>
When the retina receives the image, as light rays bounce of from objects and enter the eye, the image is smaller and upside down. So from our eyes gather a completely inverted view of the world. Then it is the brain that corrects the image and we see the world as an “interpreted image” of our visual cortex. We see from our brain-Eighteenth century German philosopher Immanual Kant, who drew a clear distinction between the forms that appear in the mind–what he called the phenomenon (a Greek word meaning "that which appears to be")–and the world that gives rise to this perception, which he called the noumenon (meaning "that which is apprehended"). All we know, Kant insisted, is the phenomenon. The noumenon, the "thing-in-itself," remains forever beyond our knowing. So in essence, regardless of the complexity of our Optical Nervous system, the the nature of Reality remains beyond our perception, as our brain can only at best interpret data given by the senses and compute the most plausible picture in a tiny dark corner at the back of our head. Too small a place to contain the real nature of the world around us. Hence its perfect in serving our daily life requirement but fails miserably when faced with questions outside its domain.
Open your eyes, and you are met with an array of different colours, but amazingly you can only detect three different wavelengths of light, corresponding to green, blue, and red. Combining these three signals in the brain creates millions of different shades
Each eye has between 6 and 7 million cone cells, containing one of three colour-sensitive proteins known as opsins. When photons of light hit the opsins, they change shape, triggering a cascade that produces electrical signals, which in turn transmit the messages to the brain. Well over half of our cone cells respond to red light, around a third to green light, and just two per cent to blue light, giving us vision focused around the yellow-green region of the spectrum.
The vast majority of the cone cells in the human eye are located in the centre of the retina, on a spot known as the fovea, measuring just fractions of a millimetre across. Light is focused on this point, providing a crisp, full-colour image at the centre of our vision. The remainder of the retina is dominated by 120 million rod cells, which detect light, but not colour.
We are so used to seeing the world in red, green and blue that it might seem strange to think that most other animals cannot, but three-coloured vision like our own is relatively unusual. Some species of fish, reptiles and birds have four-colour vision, able to see red, green, blue and ultraviolet or infrared light, but during mammalian evolution, two of the four cone types were lost, leaving most modern mammals with dichromatic vision – seeing in shades of just yellow and blue.
Some snakes have infrared vision
This was not a problem for many early mammals, because they were largely nocturnal, and lived underground, where there was little need for good colour vision. However, when primates started moving into the trees, a gene duplication gave some species the ability to see red, providing a significant evolutionary advantage in picking out ripe red fruit against the green leaves.
Even today, not all primates can see in three colours; some have dichromatic vision, and many nocturnal monkeys only see in black and white. It is all down to environment; if you don’t need to see all of the colours in order to survive, then why waste energy making the pigments?
Our eyes are only able to produce two-dimensional images, but with some clever processing, the brain is able to build these flat pictures into a three-dimensional view. Our eyes are positioned about five centimetres (two inches) apart, so each sees the world from a slightly different angle. The brain compares the two pictures, using the differences to create the illusion of depth.
Safiyyah Sabreen studied Mechanical Engineering and is currently pursuing her Master's in Philosophy. She is the Content Director for KNOW. Being interested in the field of Islam and Science and Islamic Eschatology, she produced a documentary on the Golden Age of Islam and directs the Second Golden Age series.