The Human Eye

Hello Everybody!

After a very interesting lesson about the human body, I would like to get with you a little bit deeper in the topic. As all of you probably know, we, humans, have an amazing opportunity to perceive the world in five different ways, commonly known as five senses. Today I would like to present you some information about one of them; the one that let’s us see the world – sight. To use it, we need eyes, and that will be the topic of our today’s lesson. So let’s take a closer look at the human eye!

Here you have a text that will explain you things related to the eye. I want you to read it carefully and then go to the link below to answer some questions related to the information you will get from the text. Hope you will enjoy it!

How does it look like?

The eye is about 2.5 cm in length, weighs about seven grams and is shaped roughly

like a ball. It sits in a special place in your skull called the eye socket. When you look in the mirror you can see several parts of your eye. The white part is called the sclera. The sclera is very tough and covers most of your eye. If you look very carefully you can see little red threads, which are tiny blood vessels bringing oxygen to the sclera. If you rub your eyes a lot, some of the tiny vessels break, and that is why your eye sometimes looks a little pink. The sclera covers the colored part of your eye as well, but here it is clear, and has a different name: the cornea. The cornea is very important because it lets light come into your eye. It also helps the eye to focus as light passes through. The colored part of your eye is called the iris. In the middle of your iris is the black pupil.

Light first travels through the cornea, and then into the pupil. There are tiny muscles

attached to the iris that help control the amount of light that can get through. In very

bright light, the pupil will get smaller, and when it is darker, the pupil will get bigger, just like a camera lens. Between the iris and the cornea is the anterior chamber. Anterior means “front”. The anterior chamber is filled with a clear liquid called the aqueous humor. The aqueous humor provides oxygen, proteins and glucose (sugar) to the eye. After light passes through the pupil, it passes through the lens. The lens is roughly the shape of a squished ball, and it is completely clear. The lens’s job is to focus the stream of light coming through the pupil onto the back of your eye, which is called the retina.

The lens is attached to a muscle called the ciliary muscle. This muscle causes the lens to actually change shape as you focus on things! When you look at something very close to you, the lens gets thicker, and when you look at something far away, the lens gets thinner!

Between the lens and the retina is a gel-like clear material called the vitreous humor. The vitreous humor is important because it gives the eye its shape.

How do we see?

The back of the eye is like a movie screen for the images you see. The retina is made up of very specialized cells called rods and cones. There are about 120 million rods and about 7 million cones in each eye! Rods cannot sense things in color, but they can see black and white and different shades of gray. After the sun has set, when you can no longer see anything in color, the rods are working very hard. Rods also allow you to see the shape of different objects. There are three different types of cones that work in bright light: red, blue, and green. Together, these cones process the light waves that come into the eye and let you see the many different colors you do. The rods and cones send all the information they gather through the optic nerve at the back of the eye. The brain then uses the nerve signals to put together a picture of the outside world. The place in the retina where the optic nerve exits is called the blind spot. This is because there are no rods or cones in this area, and if an image is projected onto this part of the retina, you cannot see it.

The eye’s self - protection

The eye also has a couple of ways to protect itself. The biggest protection for the eye is the eyelid. The eyelid helps to keep the eye clean and moist. Opening and closing the eyelid is called blinking, and you can do it when you want to and without even thinking about it. This means that blinking is both a voluntary and involuntary action. Your eyelids will shut automatically to stop things from getting into your eye.

Don’t forget your eyelashes! They form a team with your eyelids to keep dirt and dust out of your eye. But what happens when a piece of dust does get in your eye? That’s when tears come in to play. Tears come from glands called lacrimal glands which are located just above the outer corner of your eyes. Every time you blink a small amount of tears is released into your eye to keep it moist. Tears also help to wash away bacteria and dust from the surface of your eye. Tears drain out of your eye by entering the lacrimal duct or tear duct.

Ocular defect

Some people wear glasses; do you know why? Sometimes people have trouble seeing and glasses help them. When you have your eyes tested, the doctor asks you to read letters from different lines of an eye chart. If you have normal vision, you have 20/20 vision. This means that you can read line twenty at twenty feet away. If your vision is 20/200, this means that you can read the same letter at 20 feet that a normal eye can read at 200 feet! A person with 20/200 eyesight is legally blind in the United States.

There are many other degrees of vision between 20/20 and 20/200, and some people even have better than 20/20 vision. People wear glasses for different reasons. Some people cannot see things very far away, and they are called nearsighted (myopic). In these people, the image is focused in front of the retina. Some people have trouble seeing things that are close, and these people are called far-sighted (hyperopia). In these people the image is focused behind the retina.

Glasses help to bend the light enough so that the image is always focused on the retina.

Now I would like you to answer the question in the exercise below, to see how much you understood from the text. Hope you learned a lot of interesting things! And remember - your eyes are doing a very importnant job! Take care of them, because the eyes you have now are going to be with you forever, helping you see the world!

How do we see