In our current blog series, we have so far covered the parts of our eyes that comprise the so-called optical part of the eye. You can read more about the cornea here and about the lens here. We will not follow the light further into the eye in today’s blog, but we will just summarize the important bits for refraction from the previous blogs.
. We will not follow the light further into the eye in today’s blog, but we will just summarize the important bits for refraction from the previous blogs.
What is refraction?
Well, the term refraction basically describes the process of light bending and focusing it exactly onto the retina. Only if the light is correctly bent and focused onto the retina, we can perceive the clear image. If, however, this process is impaired for whatever reason, the refractive errors ensue. We will talk more on refractive errors later in today’s blog.
The light is firstly bent by the cornea and then by the lens. Total light bending power of the human eye is about 60 D (dioptres). About 43 D contributes the cornea and further 10-20 D contributes the lens. Click here if you have not read yet, how the lens can change its bending power. In addition to the optical bending power of the eye, which is the function of the cornea and the lens, there is also another important bit that defines whether we can see clearly of not. It is the axial eye globe length. Humans are born with eye globes that are about 16 mm (0.6 inches) long. During our growth the eye globe grows as well. However, due to various different reasons (which we might cover in some blogs in the future), the axial lengths of the adults’ eyes vary very much between individuals. In fact, whereas the mean axial length is about 23 mm (0.9 inches), it can vary between 18 and 29 mm (0.7 and 1.1 inches) in otherwise healthy eyes. This leads us to aforementioned refractive errors.
Refractive errors due to eye globe deformation
Firstly, we will deal with refractive errors that are the consequence of either too short or too long eye globe for the refractive power of the eye. Let us assume, that the refractive power of human eyes is about 60 D and does not vary much between individuals. Those that have the eye globe that is too long will suffer from myopia (nearsightedness) and those that have the eye globe that is too short will suffer from hyperopia (farsightedness). The light will focus in front of the retina in the former and behind the retina in the latter. Consequently, both will suffer from impaired visual acuity. Thankfully, correction is easy and straightforward. Corrective spectacles or contact lenses with plus dioptre for those with hyperopia and minus dioptre for those with myopia.
Astigmatism and presbyopia
Nevertheless, blurry vision is not only due to the improper eye globe lengths or inappropriate optical bending powers. Apart from already mentioned myopia and hyperopia, there are two more common refractive errors. The first one is astigmatism and the second one presbyopia. Astigmatism is a cylindrical refractive error, that is the consequence of corneal irregularity. The optical bending power is too weak or too powerful across a certain meridian of the cornea. Imagine that the cornea is not perfectly round as a basketball but is slightly curved such as a rugby ball (or football for our American readers). The correction, however, is simple. Cylindrical or toric corrective lens (either spectacles or contact lenses) oriented to the corresponding axis of corneal astigmatism. Presbyopia on the other hand, is the refractive error that probably none of us will avoid. It is a normal process that starts at around 35 years of age and worsens progressively with age. Due to ageing, our lens gradually loses its flexibility so that the accommodation is slowly but surely becoming insufficient. The consequence is the blurry vision in the near field, especially when reading. As with most refractive errors, the correction of presbyopia is rather easy as well. The so-called reading glasses with plus dioptres are usually all one would need.
Stay tuned and subscribe!
We have now mostly covered the optical part of our eyes. In the next blog, we will talk about the receptive part, namely the retina. Stay tuned and subscribe, so that you do not miss it!
Missed few parts of the series? Check them out:
Part I: On the Origin of Light
Part III: Iris, pupil and lens
Part V: The retina (and the vitreous)