How do our eyes see?
Post on 11th November 2015
The human eye is so complex it’s almost hard to believe that we can see at all.
In a single glance, lasting a fraction of a second, our eyes work with our brains to tell us the size, shape, colour, and texture of an object. They let us know how close it is, whether it’s standing still or coming toward us, and how quickly it’s moving.
Evolved over the course of 100 million years, our eyes function in a similar way to a camera, with an opening through which the light enters, a lens for focusing and a light sensitive membrane at the back.
Only part of the eye is visible, the whole eye is about the size and shape of a ping-pong ball. The eyeball sits in the eye socket – the orbit – in the skull, where it is surrounded by bone. The visible part of the eye is protected by the eyelids and the eyelashes, which keep dirt, dust, and even harmful bright light out of the eye.
Eyes are also protected by tears, which moisten them and clean out dirt, dust, and other irritants that get past the defences of the eyelashes and eyelids. Tears also help protect against infection.
To see, the eye has to move. Six extraocular muscles surround the eyeball and act like the strings on a puppet, moving the eye in different directions. The muscles of each eye normally move together at the same time, allowing the two eyes to remain aligned.
The wall of an eyeball is made up of three layers:
1. The sclera is the outermost protective layer. This tough, fibrous tissue surrounds the eyeball and attaches to the cornea, which is the clear front surface of the eye. What we see as the white of the eye is the sclera. Over the sclera lies the conjunctiva, a clear skin layer that protects the eye from becoming dry.
2. The choroid is the middle layer that contains blood vessels that deliver oxygen and nutrients to the inside parts of the eye.
3. The retina, the innermost of the three layers, lines the inside of the eyeball. The retina is a soft, light-sensitive layer of nervous system tissue. The optic nerve carries signals from the retina to the brain, which interprets them as visual images.
The space in the center of the eyeball is filled with a clear jelly-like material called the vitreous humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape.
On the front surface of the eye is the see-through, circle-shaped cornea. You can’t see a person’s cornea the way you can see the coloured part of the eye behind it — the cornea is like a clear window that focuses light into the eye.
Behind the cornea is a watery fluid called the aqueous humor. The cornea and aqueous humor form an outer lens that bends light on its way into the eye. This is where most of the eye’s focusing work is done.
The coloured circular membrane in the eye just behind the cornea is called the iris. The iris controls the amount of light entering the eye through the pupil, which is the opening in the centre of the iris that looks like a tiny black circle.
Like a camera, which controls the amount of light coming in to prevent both overexposure and underexposure, the iris becomes wider and narrower, changing the size of the pupil to control the amount of light entering the eye. The pupil gets bigger when more light is needed to see better and smaller when there’s plenty of light.
The eye’s lens sits just behind the iris. Just like a camera lens, the eye’s lens focuses light to form sharp, clear images. Light that has been focused through the cornea and aqueous humor hits the lens, which then focuses it further, sending the light rays through the vitreous humor and onto the retina.
To focus on objects clearly at varying distances, the eye’s lens needs to change shape. The ciliary body contains the muscular structure in the eye that changes the shape of the eye’s lens. In people who have normal vision, the ciliary body flattens the lens enough to bring objects into focus at a distance of 20 feet or more. To see closer objects, this muscle contracts to thicken the lens. Young children can see objects at very close range; many people over 45 have to hold objects farther and farther away to see them clearly. This is because the lens becomes less elastic with age.
The retina (the soft, light-sensitive layer of tissue that lines the back of the eyeball wall) is made up of millions of light receptors called rods and cones. Rods are much more sensitive to light than cones. Each eye has about 125 million rods that help us see in dim light and detect shades of grey, but they cannot distinguish colours. In comparison, the 6 million cones in each eye allow us to see in bright light and they also sense colour and detail.
The macula is a small, specialised area on the retina that helps the eyes see fine details when we look directly at an object. It contains mainly cones and few rods.
When focused light is projected onto the retina, it stimulates the rods and cones. The retina then sends nerve signals are sent through the back of the eye to the optic nerve. The optic nerve carries these signals to the brain, which interprets them as visual images. The portion of the brain that processes visual input and interprets the messages that the eye sends is called the visual cortex.
As in a camera, the eye’s lens transmits light patterns upside down. The brain learns that the impulses received from the upper part of the retina are really from the lower part of the object we’re seeing and vice versa.
Most people use both eyes to see an object. This is called binocular vision, and images are formed on the retina of each eye. These images are slightly different because the object is being viewed from slightly different angles. Nerve signals representing each image are sent to the brain, where they are interpreted as two views of the same object. Some of the nerve fibres from each eye cross, so each side of the brain receives messages from both eyes. Through experience, the brain learns to judge the distance of an object by the degree of difference in the images it receives from the two eyes. This ability to sense distance is called depth perception.
As you can see the eye is an incredible organ and one we should treasure. Regular eye checks are a simple way to keep your eyes in good working order. To make an appointment visit our home page.