Beautiful oceans, mountains, the sunset or other people’s smiles. All of that can only be perceived with our two eyes. Often, we take it for granted to be able to see our favourite people every day and don’t know a lot about the science behind it. In the following article, I will explain the fundamental way our eyes work and why we might see the world differently than others.
Autonomy of the eyes
The eye is made out of many different parts which all have to work together in order for us to see an image clearly. The process of seeing starts with light entering through the cornea, which is the clear outer part of the eye. In the centre of the iris, the part of the eye that gives everyone a unique eye colour, there is an opening which is called the pupil. After the light has passed freely through the cornea, the amount can now be controlled by the pupil. You might have already noticed that the pupil, the black part of the eye, gets smaller when it’s bright outside and bigger when being in a dark room. By enlarging the pupil, the eye can take in more light, and by shrinking, it can protect the eye from very bright lights which could damage them. Because the front part of the eye is curved, the light bends while entering and appears upside down on the retina. Later, the brain can turn the image the right way up. In the next step, the light has to pass the crystalline lens which is a clear structure which refracts the light to focus it on the retina. The lens is able to change its shape with the help of the ciliary muscle. That way, the eyes can focus on something from different distances to create a clear image for us. The vitreous fills the globe of the eyeball and helps it to hold its shape. Once the light reaches the retina, it turns it into signals about the image and sends them through over a million of nerve fibres to the optic nerve. The back part of the retina is the only part that is light sensitive, it’s around the size of a 50-cent coin and it contains photosensitive cells. There are two different kinds of cells, the rods and the cones. The cones are responsible for daylight vision and they are split into three kinds reacting to different wavelengths of light; red, green and blue. The cones are the reason why we are able to see in colour, without them, we could only see the world the way we do at night. The rods are the cells responsible for said night vision. They are sensitive to light, but not to colour, which is why you can’t see colours at night and everything appears to be black and white. The last step is for the electric signals to travel from the optic nerve to the brain, where we can then perceive the image.
Good to know; the aqueous humour is a liquid behind the cornea which is clear and fills up the space in front of the lens. The sclera is a white layer which protects the eyes and is responsible for letting the eyes appear white around the iris.
Why do we see differently?
There are many different reasons why our eyes might not work correctly; short- and long-sightedness are only two of them. In total, over 30% of humans are affected by those two most common eye problems. Short-sightedness or Myopia is a disorder found in 22% of people and it’s caused by the light focusing in front of, and not on the retina. That is why; objects far away can appear blurry while those closer look normal. Most of the time, this happens because the eyeball grew too long and that’s why the focus on the retina can’t work correctly. Long-sightedness on the other hand can be seen in 10% of the world’s population and is also known as hyperopia. The people affected by it may perceive things close by as blurry but can focus clearly on things further away. Hyperopia is dependent on the size and shape of the eye, just like Myopia. Other than with Myopia, the eyes of people living with Hyperopia are either too short or the cornea is not curved enough. This results in the light focusing behind the retina and not on it.
As you can see there goes a lot of work into our ability to see our surroundings with or without the support from glasses!
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