Optics and the human eye

I bought myself an optics kit as a Christmas present. It is awesome because it has lots of lenses, mirrors and lasers. I have been learning lots with it and I will be posting a few blog posts about this topic. I don't to spoil it but I will show you how to build your own telescope.

But for today I will teach you what I have learnt about the human eye.

There are two things that control the amount of light that enters your eye:  the eye lids and the iris. I don't need to explain you what the eye lids are, because I think you already know. The other one is the iris which is the coloured part of your eye. One thing I learnt about it is that the pupil is just a hole in the iris, that gets bigger or smaller to let in the right amount of light. So it is not a thing by itself, it is just a hole in the iris. It is black because it is very dark inside. A curiosity of mine is that every human's iris is different, which is why it can be used for recognition.

Then, there are two parts of the eye that help you focus. They us refraction to bend light, just like the lenses in my kit so they are the optical part of the eye. On the outside we have cornea. The cornea has a fixed shape so it always refracts the light by the same amount. But then after the iris we have the lens, which unlike the iris it can change shape with the help of some muscles. This allows you to focus the incoming light so that they always come together at the very back of your eye. The back of your eye is called the retina and it is home to your photoreceptors

There is two types of photoreceptors. The rods help you see in dim light because they are very sensitive, however, they can only see in black and white. This is the reason why you can't see colours in the dark. The other photoreceptors are the cones which allow you to see red, green and blue light but these need more light to get activated. An interesting fact is that there 17 times more rods than there are cones. All the signals that these guys generate go to the brain via the optic nerve.

I have learned that there are three main visual disorders that humans can have. They are jointly called ametropia, which means "unable to focus":

- Myopia or short-sightedness often occurs when the eyeball is too long and the image forms before the retina. This results in far away objects being blurry. People with myopia wear glasses with divergent lenses so that the image forms later. I have simulated this with my optics kit and real glasses

- Hyperopia is the opposite of myopia. The eyeball is too short and the image forms behind the retina so we need glasses that help the image to converge sooner

- Astigmatism. This is where the eyeball is not symmetric. Many people have this in additional to myopia or hyperopia

I was on holidays last year in Singapore and I noticed that many children were wearing glasses. So I decided to ask Google about the percentage of children with myopia in Singapore versus Australia. It turned out that in Australia there is only 28% of children that have myopia compared to 65% in Singapore. I also read that one of the main causes of myopia is the lack time outdoors. And with that I am going to wrap up today's blog post, get my ball and head to the park because I don't want to end up with glasses.

See you next time!