The human eye is the most wonderful gift we've ever got. Eye helps us see almost everything around us, except of course air, atoms or small molecules. Our eye works like a camera and captures images of what we see and then send those images to the brain. The brain then functions the image and only then are we able to comprehend the meaning of the image, just like you are doing right now.
The human eye is a sensory organ which is a part of the sensory nervous system and is an organ of vision. It reacts to visible light which penetrate through it and forms an image on to the retina or the screen of the eye. Now, we'll explore more on how we see things after we understand what are the parts of the eye.
Schematic labelled diagram of the human eye:
That's a lot of stuff present in our eye, isn't it? Let us now understand the functions of each of the element of the eye.
1) Cornea: The cornea protects the eye from foreign agents. It is present in the outermost layer of the eye. Now, the interesting thing about cornea is it refracts most of the light (around 75%) before entering the eye. Refraction in simple terms is the bending of light in a different medium. To understand more on refraction, visit why does light bend when it enters a new medium. Now, most of people get confused with the white portion of the eye as cornea. But cornea is actually transparent!
2) Sclera: The white portion of the eye is called the sclera. It functions as the supporting wall of the eyeball. It helps maintain the shape of the eyeball and protects it from any injury. The sclera is covered by conjunctiva, which are visible mucus membranes that lubricate or moisturize the eye. An infection in the conjunctiva leads to pink eye or conjunctivitis where the sclera looks all reddish pink in color.
3) Aqueous Humor: The aqueous humor is present behind the cornea and has a water like consistency. Its main function is to maintain pressure balance in the eye.
4) Iris: Iris is a muscular diaphragm which controls the size of the pupil. The iris can regulate the amount of light entering the eye. As it covers the hole or pupil where light can enter, the iris can contract or relax according to the intensity of light thus controlling the amount of light entering. Also, it determines the color of the eye. If you see people with different eye colors such as blue, green, brown, grey etc., it is actually the color of their iris.
5) Pupil: The black hole through which light enters is the pupil. It is not that gravitational monster lurking in space, but the small hole from which light does not come back and thus looks black in color. The pupil is simply a hole through which light penetrates. That's its only job.
6) Crystalline Lens: We all know that lenses refract light or bend its path. The same way, crystalline lens also refracts the light entering through the pupil and focuses the image on the retina. We already saw that the cornea refracts most of the light entering the eye. Hence, the crystalline lens refracts the left over light to a focus to form a sharp image on the retinal screen. It is a biconvex transparent lens which has the ability to change its shape according to the distance of object from the eye to form a sharp image.
7) Ciliary Muscles: Ciliary muscles are one of the most important eye parts which functions to adjust the crystalline lens. They have the ability to change the focal length of the eye. Now, focal length (in this case) is the distance between the retinal screen and the crystalline lens. Ciliary muscles can contract or relax accordingly to change the thickness of the eye lens in order to accommodate the eye to form a sharp image. They are present on either ends of the lens and control the shape of the lens. Any malfunctioning in the ciliary muscles could lead to improper or blurry vision as the lens may not work properly.
8) Vitreous Humor: The vitreous humor as you can see, occupies a large portion in the eye. It has a honey like consistency and also refracts a little portion of the light entering the eye. Its main function is to maintain the spherical shape of the eye ball. It also prevents the eye from collapsing.
9) Retina: The retina is another such important part of the eye which acts as a screen where the image forms. All the light entering our eye gets focused onto this screen called the retina. It is a photoreceptive screen as it captures light (photo) to form an image. Retina has two special parts known as rods and cones which are basically muscular cells which react to the light. Rods are cylindrical photoreceptors which sense the intensity of light. Rods help us see even in the darkness or in areas of dim lights or during night times. Whereas cones are the reason why we see our world colorful. Cone cells sense colors and the colorful world we see around us is because of the presence of cone cells in our eye. People who have significantly lesser cone cells tend to have color blindness. And people with significantly lesser rod cells tend to have night blindness.
10) Optic Nerve: Optic nerve acts as the messenger of the eye and sends signals to the brain which are then converted into electrical signals. It connects the eye to the brain. When the brain receives the signals, only then do we understand the image.
Now, there are two types of spots found in our eye. The yellow spot and the blind spot.
11) Yellow Spot: It is an oval spot at the center of the retina. The spot or macula lutea is activated when an eye is looking directly at an object. It is the part of the retina that is responsible for sharp and detailed central vision.
12) Blind Spot: The blind spot is basically the point of the entry of the optical nerve on the retina that is insensitive to light. Each eye has a small portion of the blind spot where there are no photoreceptors. It is also known as scotoma.
Let us understand how we see things around us. When light hits an object from a source and reaches our eye, most of it is refracted by the cornea and then enters the aqueous humor of the eye. It then enters inside the eye through the pupil and hits the crystalline lens. The lens refracts the light even more and focuses the light ray to form a sharp image on retina. The retinal cells i.e. rods and cones sense the light and the optic nerve then sends the signals to the brains for us to see the image completely. Now, the image that is formed on the retina is inverted in reality. Hence, the image formed on the retina is actually real and inverted.
It is our brain which flips the inverted image and only then are we able to see the world around us in an upright position. Thus, the human eye helps us to see the colorful world around us.
Interesting facts:
1) The refractive index of both the aqueous humor and vitreous humor is 1.336.
2) The refractive index of the cornea is 1.376.
3) When there is a lot of light entering the eye, the iris expands and makes the pupil smaller in size so that small amount of light enter our eye and does not damage it. And when very dim or less amount of light is available, the iris contacts and makes the pupil as large as possible so that it could consume as much light as possible and help us to see clearly. So useful! This is why when light is suddenly switched on after a long time in darkness, our eye hurts or we see completely nothing for about a few seconds.
Surely, the human eye is the most wondrous gift we've ever received from the nature.
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