We are able to see because of the presence of a light source, be it sun light or an artificial light source. Objects are visible to us because light rays from a source are reflected on these objects and then into our eyes. This is why we can't see in complete darkness—in the absence of a light source.
A basic knowledge of the anatomy of the human eye will help you understand the anatomical terms used throughout this article.
How Light Enters the Eyeball
Light from the outside world enters the eyeball through the transparent anterior part of the eye called the cornea. Light then passes through the pupil (the black circle in front of the eye) and the anterior chamber filled with a fluid called aqueous humour, then through the crystalline lens and the posterior chamber filled with a fluid called vitreous humour. From the posterior chamber, light is focused onto the retina—the back wall of the eyeball where 'seeing' starts to happen. The cornea and the lens are the two main structures responsible for focussing light onto the retina.
How Signals Travel to the Brain
The retina contains millions of cells known as photo receptors, classified either as rods or cones. Rods help us see black, white and different shades of gray while cones helps us see color. Rods and cones convert shapes and colors into nerve impulses (messages) which are sent to the brain.
Millions of nerve impulses from the retina converge at the optic nerve, where they are carried along to the brain for processing. The brain translates the messages into meaningful images.
In summary, light that enters the eye is focussed on the retina. The retina compiles a detailed report of what the eye captures and sends it to the brain. The brain analyses and translates the report into something meaningful, such as a dog, a tree or a picture of Justin Bieber. It takes only a fraction of a second for information (light) that enters the eye to reach the brain, this incudes the processing time of the brain.
Illustration of How the Eye Works
How Major Parts of the Eye Work
The workings of the eye is similar to that of a camera. Just as the diaphragm of a camera controls the amount of light that enters the camera, the iris controls the amount of light entering the eye. Just like the camera lens, the crystalline lens of the eye is able to focus—to properly direct light, to obtain a clear image of objects in our field of vision. The retina of the eye works more or less like the photographic film of a camera.
The iris is made up of radial and circular muscles that involuntarily contract or relax. When the radial muscles contract, the pupil dilates (gets bigger) and allows more light to enter the eyeball. When the circular muscles contract, the pupil constrict (gets smaller) and allows less light to enter the eyeball.
The iris is also the most colorful part of the eye. It is what displays a person's eye color.
Depending on whether you are looking at a far away or near by object, the lens continuously changes its shape to regulate the focus of light on the retina. When you are looking at a far away object, the ciliary muscles relax and the lens becomes thinner. This is because light from far away objects need less refraction (bending) to be properly focussed. When you are looking at a nearby object, the ciliary muscles will contract and the lens will become thicker. Light from near by objects require more bending to be properly focussed.
The crystalline lens of the eye gradually loses its flexibility with age. This largely explain the strong correlated between age and vision problems. The loss of flexibility makes it difficult to attain the shape necessary for the lens to properly focus on nearby objects. This condition is called presbyopia. It is associated with blurred vision when viewing nearby objects. Age is therefore an important factor affecting how the eye works.
Light entering the eye is first refracted by the cornea, and then refracted a second time by the lens before it reaches the retina. This double refraction of light (by the cornea and lens) causes the images to be focussed upside down on the retina. The brain however interprets the image in the correct perspective. In addition to translating the nerve messages into meaningful images, the brain also presents the images in the correct orientation—right side up.
Adaptation of the Eye
The human eye is sensitive to changes in the light intensity of its surrounding and is able to adapt to these changes.
In a bright sunny day, if you suddenly step into a dark or poorly lit room, you would experience a sudden significant depreciation of vision. Your vision would gradually return to normal after spending several minutes in the room. This process where by the eye adapts to a sudden steep decrease in the light intensity of its surrounding is known as dark adaptation. The pupil plays a key role in dark adaptation, it dilates to allow more light to enter the eye.
Similar to dark adaptation, stepping into daylight from a dark or poorly lit room would trigger a sudden depreciation of vision. Your vision would slowly return to normal after a few minutes. This process where by the eye adapts to a sudden increase in light intensity is known as light adaptation. During light adaptation, the pupils constrict to reduce the amount of light entering the eyeball.
Why We Blink
When we blink, our eyelid covers the surface of the eyeball with a thin film of tear. This provides a smooth, clear and moist surface that helps the cornea maintain maximum refraction of light to the lens. Tear also contains antibacterial properties that protect against microbes. In addition, the eyelids and eyelashes help prevent tiny particles (including insects) from entering the eyes.