In the previous blog we have covered what the light is and where it comes from. Should you not have read it yet click here. On the contrary to a hundred million miles we have covered in the previous blog, this time we will only go on for one more millimetre (five hundredths of an inch). The light has been stripped of most of the harmful radiation when it finally reaches the eye.
Through the tear film…
Firstly, it goes through a tiny layer of fat and water on the surface of the eye – the tear film. You might think that the latter is not at all important, but it is. It is indeed very much important as variations in the tear film can lead to dry eye disease. This is a very frequent disorder that affect up to 20-25% of population and is experienced as persistent discomfort or pain in the eyes. Treatment involves a plethora of different eye drops, some of which are less some more effective. It is also important that people suffering from dry eyes restrain from excessive looking onto computer/tablet screen as it is established that looking onto the screen decreases the frequency of blinking which then makes the dryness of the anterior surface of the eye even worse.
… to the cornea!
Moving slightly inwards, the light then reaches the cornea. The cornea is a transparent shield of the eye that covers the lens, pupil and iris underneath. Its main job, apart from the protection of deeper structures, in that acts as a lens, which means it bends and focuses the light onto the retina. Yes, you have heard that right. In our eye, we also have the lens that does a lens job (focusing the light onto the retina) but the cornea does ever so slightly more so. Confusing? Do not worry we will explain what the main job of our lens is in later blogs.
The cornea, which is about half a millimetre (two hundredths of an inch) thick, is also one of few tissues in our body that are completely without blood vessels and the tissue that is by far the most innervated (has the most nerves per square millimetre) of all tissues, 20-40 times more than teeth and 300-600 times more than skin. That is why it is so sensitive to touch and that is also why all corneal injuries hurt so much. Being the shield of the eye, it is very much susceptible to injuries and infections which are also among more frequent diseases that affect cornea. If not addressed properly, some of them can cause corneal haze that impair vision and sometimes even require corneal transplantation in order to restore visual acuity. Contact lens wearers are especially at risk for corneal infection, so careful handling and maintenance of contact lenses are crucial in order to avoid them.
What do the lens and iris have to do with vision? Coming soon!
In the next blogs we will first follow our light path further into the eye and cover the role of the lens and the iris. Then we will talk about the refractive errors and how to correct them. Later, we will reveal how the light is converted to electricity in the retina and how are we able to perceive those signals as vision.
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Missed few parts of the series? Check them out:
Part I: On the Origin of Light
Part III: Iris, pupil and lens
Part IV: Refraction and the Eye Globe
Part V: The retina (and the vitreous)