The Cornea: Casting a Wide Net
If you want to catch a whole lot of fish, it’s best to bring a really big net, right? As a majority of species’ eyes evolved, nearly all took on the same approach, and the same holds true with humans. The dome-shaped lenses of our eyes are known as the corneas. The unique shape of the cornea allows it to collect and transmit images and light at wide angles.
The Pupil: It’s Really Nothing
Most of us are so familiar with the way eyes appear that we just accept the pupil as another physical feature of the eye. In reality, the pupil is nothing; only a hole underneath the cornea acts as a tunnel for light travelling to the retina. The reason your pupils are black in appearance is due to most of the incoming light being absorbed by the retina and the surrounding tissue.
Sclera sounds like a word shouted at a dramatic turning point in a Greek tragedy, but uninterestingly, sclera simply translates to “hard” or “tough” and describes the white, fibrous tissue surrounding your eye’s retina. When the eye is under pressure or receives some form of physical impact, it is this tough tissue that comes to your eye’s rescue. Sclera!
The Iris: Those Beautiful Baby Blues (and Browns, and Greens, and Hazels...)
The iris is the ring around the pupil that gives eyes their signature colour. While many think the iris is there to make our eyes more attractive (more on that later), the colourful melanin found in the iris actually plays a huge role in controlling the amount of light passing onto the retina.
One weird thing is that for the vast majority of human history, everyone had brown eyes or a variation thereof. But, around 10,000 years ago, one individual (let’s just call him “Mr. Batty-Eyes McStudmuffin”) was born with a single genetic mutation that reduced the amount of melanin in his irises, giving them a more opaque, blueish appearance. So, how did the genetic abnormality of blue eyes spread so quickly among European populations? To answer this question, we’ll turn to John Hawks, professor of anthropology at the University of Wisconsin–Madison, who explained it quite succinctly: “This gene does something good for people. It makes them have more kids.” Thanks, Professor Hawks.
One great takeaway from this is that if you’ve been jealous of a friend or sibling’s gorgeous blue eyes for all these years, you can, with confidence, refer to them as a “mutant.”
The Inner Lens: Seeing the Big Picture (by Shrinking it Down)
The inner convex lens of the eye helps to bend and condense incoming light rays so they can easily pass through the retina. Without this lens, a lot of visual information would be lost, and our visual abilities would be greatly narrowed.
The Retina: The Eye’s Light Sorter
After light passes through the lens, it reaches the retina. At the centre part of the retina is an area that is known as the fovea centralis. It is this cluster of “cone rods” that captures red, green, and blue light waves, as well as varying degrees of light and darkness. The fovea centralis is also responsible for transmitting focused visual tasks, such as relaying the paragraph you just read to your brain.
Notice how we only mentioned that your retina’s cone rods only collect the colours red, green, and blue? The cone rods in your retina are only designed to react to just these three combinations of colours. So what about yellow and gold hues? Absolute figments; yellow is a colour your brain’s occipital lobe simply makes up because it knows you would be confused by seeing objects that are simultaneously red and green. Yes, yellow is actually red and green light combined. Similarly, the things we perceive as “white” are actually all the colours of the visible light spectrum reflecting off an object.
Remember that black and blue (white and gold) dress controversy a few years back? The unique lighting and colours of that picture, and the way the light reflected off the dress, were just the right conditions to throw off roughly half the population’s cone rods. This is just one of many examples demonstrating how deceptive human eyesight can actually be.
One Last Thing...
In the introduction, we mentioned how humans can see only one thing during their whole existence — photons. This is partially true, but it’s not the whole story. While we can only detect a limited band of light waves, the way we actually visualize the world around us is better explained like this: the retina’s cone rods become stimulated by and react to incoming light. In reaction to this stimulus, the cone rods create chemical signals that are quickly converted into electrical signals. These signals then travel along the optical nerve fibres until they reach the occipital lobe, where they are translated into the colours and everything we “see.” The whole process, from light capture to interpretation, takes only 13 milliseconds.
Since the visual cortex (and other portions of the brain) is active while we dream, we can still visualize many things even though they are not actually happening. Interestingly enough, the least-active areas of our brains during dream activity are those controlling logic and reason.
Thank you for joining us in exploring these fascinating topics. If you have any questions about your vision and keeping your eyes healthy, please feel free to reach out to your local optometrist in Victoria or consult our Find an Optometrist guide to locate the optometrist nearest you.