How the Eye Works
How Does the Eye Work?
When we look at something, we are able to see it because of the presence of a light source, be it sun light or an artificial light source. Objects are visible around us because light rays from a source are reflected on these objects and then in to our eyes. We can't see in complete darkness because of the absence of a light source.
Consider reading about the anatomy of the human eye to have a good understanding of anatomical terms used throughout this article.
How Does Light Enter the Eyeball?
Light reflected from the outside world enters the eyeball first through the transparent cornea. Next, light passes through the anterior chamber (filled with aqueous humour), the crystalline lens and then the posterior chamber (filled with vitreous humour). From the posterior chamber, light is focused on to cells of the retina known as photo receptors. Cells of the retina convert light in to nerve impulses.
Presbyopia | Blurred Vision of Nearby Objects
Why do people tend to need reading glasses when they get older?
As people age, the crystalline lens in their eyes gradually looses its flexibility. Without this flexibility, the lens has difficulty to attain the shape necessary to focus on nearby objects. Therefore, age is an important factor affecting how the eye works.
How Do Signals Travel to the Brain?
Nerve impulses from retina converge at the optic nerve. From the optic nerve, the impulses travel to the brain where they are compiled in to images for us to see. It takes only a fraction of a second from the time light to crosses the cornea till when it reaches the brain as nerve impulses (including the time the brain makes sense of images for us to see).
How Do the Major Parts of the Eye Work?
How the eye works is similar to the way a camera works. Just like the iris, the diaphragm of the camera controls the amount of light entering the camera. Just like the crystalline lens, the camera lens is able to auto-focus far away and nearby objects. The screen of the camera that contains the photographic film is similar to the retina of the eye.
The iris is made up of radial and circular muscles that involuntarily contract. When the radial muscles of the iris contact, the pupil dilates and allows more light to enter the eyeball. When the circular muscles of the iris contract, the pupil constricts and allows less light to enter the eyeball.
Depending on whether you are looking at an object far away or near by, the lens continuously changes its shape to regulate the focus of light on the retina. When you look at a far away object, the ciliary muscles will relax and the lens will become flatter. When you look at a nearby object, the ciliary muscles will contract and the lens will become rounder in shape (to bend light more for near focus).
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. That is, the image will be turned right side up by the brain.
Illustration of How the Eye Works
Why do we blink?
Tear plays an important role to how the eye works. It contains antibacterial properties that protect the eye against bacteria. When we blink, our eyelid covers the eye surface with a thin film of tear. This provides a smooth, clear and moist surface which helps the cornea to maintain maximum refraction of light to the lens. The eyelids and eyelashes also help protect the eye from tiny particles such as dust.
Adaptation of the Eye
The eye is sensitive to changes in light intensity of light in the surrounding and is able to adapt to these changes.
- Dark Adaptation
In a bright sunny day, if you suddenly step in to a dark or poorly lighted room, you will notice that you can hardly see anything. However, after spending about half an hour in the room, your eyes gradually adapt to the darkness and you are able see objects around. This is known as dark adaptation. Part of how the eye works in dark adaptation is that the pupils dilate to allow more light to enter the eye.
- Light Adaptation
Similarly, after spending some time in a dark or poorly lighted room, when you suddenly step outside in a bright sunny day, you can hardly see anything. However, after spending about half an hour outside, your eyes gradually adapt to the light and you are able to clearly see objects around. This is known as light adaptation. Part of how the eye works in light adaptation is that the pupils constrict to allow less light enters the eyeball.