The Human Eye

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The human eye is an essential organ, which interacts with light and is necessary for the sense of sight or vision. There are two kinds of cells in the eye, which are rods and cones. 

Conscious light perception color differentiation and perception of depth are done by these cells. The human eye can differentiate between about 10 million colors, and it can also detect a single photo. The human eye is a part of the sensory nervous system.

The eyes of all mammals have a non-image-forming photosensitive ganglion in the retina which receives light, adjusts the size of the pupil, regulates the supply of melatonin hormones, and also entertains the body clock.

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(The human eye, showing a white sclera with some blood vessels, the black pupil and a green iris)

Human Eye Diagram

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(The structure of the human eye is shown above in the image.)

Parts of the Human Eye

  • Pupil: The pupil is a small opening in the iris. The iris controls the size of the pupil. The pupil’s function is to adjust the amount of light entering the eye.

  • Sclera: The outer covering of the eye is called sclera. It is a protective tough white layer (white part of the eye).

  • Cornea: The transparent part in front of the sclera is called the cornea. Light enters the eye through the cornea.

  • Iris: It is a dark, muscular tissue and ring-like structure present behind the cornea. The color of the eye is due to the color of iris. The iris regulates the amount and intensity of light entering the eyes by adjusting the size of iris.

  • Retina: It is the light-sensitive layer which consists of nerve cells. Its function is to convert the images formed by the lens into electrical impulses. These electrical impulses are then transmitted through optic nerves to the brain.

  • Lens: The transparent portion situated behind the pupil is called the lens. The lens alters the shape to focus light on the retina, with the help of ciliary muscles. It becomes small to focus objects at a distance and becomes big to focus nearby objects.

  • Optic Nerves:  You can find two types of optic nerves, which are cones and rods.

  1. Cones: Cones are the nerve cells that are more sensitive to bright light. Cones help in central and color vision.

  2. Rods: Rods are the nerve cells that are more sensitive to dim lights. Rodes help in peripheral vision.

There are no sensory nerve cells at the junction of the optic nerve and retina. Therefore, no vision is possible at this point, and it's called the blind spot.

Working of the Human Eye

The human eye operates as similar to a digital camera in several ways:

  • Light focuses mainly on the cornea, which acts like a camera lens.

  • The iris controls the light that reaches the eye by adjusting the size of the pupil, and thus it functions like the diaphragm of a camera.

  • The lens of the eye is located behind the pupil, and it focuses light. This lens helps the eye to automatically focus on near and distant objects, and also the approaching objects, like an autofocus camera lens.

  • The cornea and lens focus light to reach the retina, which is a light-sensitive zone present on the inner lining of the back of the eye.

  • The retina converts optical illusion images into electronic signals, and thus it acts as an electronic image sensor of a digital camera. These electric signals are then transmitted by the optic nerve to the visual cortex, which is responsible for the sense of sight.

Function of the Human Eye

Human eyes are a specialized sense organ which is capable of receiving visual images, thereby producing the sense of sight in us.

The eye receives direct oxygen through the aqueous humor. The aqueous humor nourishes the cornea, lens, and the iris, by carrying nutrients, removing wastes materials excreted by the lens, and maintaining the shape of the eye. 

The aqueous humor is responsible for providing shape to the eye. It must be clear to function properly.

What is the Work of the Lens in the Human Eye?

The lens is a transparent flexible tissue located directly behind the iris and the pupil. The function of the lens is to focus light and images on the retina. The cornea and the lens are responsible for focusing the image in the retina.

Due to the elastic & flexible nature of the lens, it can change its curved shape to focus on nearby or distant objects depending on the need. The lens provides around 25-35 % of the total focusing power of the eye.

The lens is attached to the ciliary muscles, which contracts and releases in order to change the shape of the lens and also its curvature.

The lens becomes oval-shaped to focus on near objects.

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The lens becomes elongated (or stretched) to focus on objects located at a far distance.

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Do You Know?

The human eye is blind for about 40 minutes every day. This is because of Saccadic masking; it is a way of the body to reduce motion blur while the object and eyes move. 20/20 is a normal vision and it's not a perfect vision. 

It means if a normal person can see an object at a distance of 20 feet, the test subject can also see the object at 20 feet.

FAQ (Frequently Asked Questions)

Q1. Do Human Eyes Grow with Age?

Ans: The eyeball grows rapidly. It has a size of about 16–17 millimetres at birth and grows upto 22.5–23 millimeters by three years of age. By the age of 12, the eyes attain its full size.

Q2. How Do We See with Our Eyes?

Ans: When we see an object, the reflected light from that object falls in our eyes, which forms the images. Thus light enters the eye through the cornea, and since the front part of the eye is curved, so the light bends thus creating an upside-down image on the retina.

Q3. Can 2 Brown Eyes Make a Blue-eyed Baby?

Ans: Yes, it is possible to produce a blue-eyed baby if both the parents are brown-eyed. However, since eye color is polygenic in nature, several other genes also exert their effect on the eye color as well. 

It is also possible to have a brown-eyed child if the parents are blue-eyed.

Q4. Why is it Called 20/20 Vision?

Ans: "Normal" vision is called 20/20. This means that the subject sees an object at a distance of 20 feet. 

20/40 vision means that the test subject sees an object at a distance of 20 feet, what a person with normal vision sees at a distance of 40 feet.