The structure & function of your eye

Eyes are exquisite features of the face; they capture the sights of the world, spark love and romance and fuel our mind’s imagination. With such a heavy dependence on our eyes, it’s important to protect and understand how they work. For centuries, humans have researched the mechanisms behind vision, which today, modern knowledge still describes as an incredibly complex chain of events inside a sensory organ smaller than a ping pong ball.

From A Ray Of Light

The eye uses light to visualize images but the range of light humans can see is only a small portion of the total light spectrum. We only see visible light, the colours of the rainbow and are not capable of seeing other parts of the spectrum like infrared, ultraviolet or x-rays without assistance.

Light beams hit an object and become refracted into our line of sight, beams then enter into the interior portion of our eyes where an optical image is made and sent to our brain. A camera needs a flash but our eyes can remarkably adjust by themselves to the varying brightness of the sun or moon light to bring an image into focus. What we see is a picture combined from the works of both our eyes which gets inverted and interpreted by our brain. Let’s take a closer look at this process below.

Look at your eyes in the mirror and you’ll see your unique coloured and patterned iris muscle. The iris controls the amount of light which enters your eye by dilating or constricting your black centred pupil. The outer white part of your eye is called the sclera and is made up of connective tissue that coats around the eyeball to maintain its circular shape. When you blink, your eyelids shut out hazards and spread tears whilst the eyelashes trap any airborne insults that might fall into your eyes.

Getting In Focus

What you can’t see is two transparent coverings: One is the cornea, which is positioned in front of the pupil and iris. This provides a passageway for light to travel through. The second is the conjunctiva membrane that covers the cornea to keep it moist by secreting a mucous.  Directly behind the pupil and iris lies the key to visual focus, the lens. Like a camera, the lens of the eye can expand or constrict to strengthen or lessen its focus power. The rays of light entering from the cornea converge and then the lens works to further focus this light towards the back of the eye. Located on the back lining of the eyeball, is a sheet of neural tissue called the retina which is covered in two types of photoreceptive cells called rods and cones.

Colour & Night Vision

There are 20 times more rods than cone cells in the eye. Cones need a lot of light to be activated and are responsible for our sharp colour vision during the day. Rods are highly sensitive to light and have less acuity, hence are used more for vision at night. So what makes day so colourful and night so different? You have three different types of cone cells (blue, red or green photo-pigment) but only one type of rod cell (with the photo-pigment, rhodopsin). When the reflected light meets our eyes during the day, each cone cell responds to their given wavelengths (red, blue or green) so that the brain can distinguish between the colours. Cones also have more neurons leaving their cells than rods thus can pass on more information to the brain. However, because cone cells need a lot of light to become activated, they are useless in the dark. At night, rod cells respond with the one photo-pigment rhodopsin, which rather than relying on colour, responds to light intensities which the brain interprets as shades of grey. In the centre of your retina is the fovea which contains a high amount of cone cells making it the point of most distinct vision compared with our peripheral vision that has fewer cones and more rod cells.

Creating Depth & Images That Last

At the retina, light is transformed into nerve impulses which are sent via the optic nerve to the brain. The nerve impulses from both eyes cross to develop depth perception. In doing so, the impulses from the left eye go to the right hemisphere of the brain and impulses from the right eye go to the left hemisphere of the brain. The optic nerve takes these impulses to the visual cortex and occipital lobe where an image is formed in your brain. The field of vision you see is 200 degrees; both eyes are synchronously used to see 120 degrees (binocular vision) and each eye independently sees a 40 degree view (peripheral vision). There are dorsal and ventral streams in the brain where the images can be taken to be stored. Visual memory helps us recall information and learn, recognise friends and family and provides our dreams with a familiar backdrop.

It’s important to understand the mechanisms behind vision so we can see how complex eyesight is and perhaps not take for granted how important it is for you to look after and protect your eyes.

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