Human Eye: The human eye is the most important sense organ since it receives light from the outside environment and provides a visual feeling. How does the human eye cause us to perceive objects? The human eye operates on the basis of visual sensation. It comprises a lens that allows us to see objects clearly, whether they are far or close to the eye. This article will teach you about the operation of the human eye and how pictures are formed in the human eye.

What is Human Eye?

The human eye is a sense organ that enables us to see things around us. We see objects because the reflected light from them falls on our eyes. The eye has a lens in it that focuses the image of the object on the retina of the eye. The photosensitive cells present on the retina of the eye detect the colour and respond to light. These cells generate electrical signals which are sent to the brain. Our brain interprets these signals and makes us perceive objects as they are.

Structure of Human Eye

The eye is one of the most valuable parts of our body. Our eyes allow us to see things around us. The eyeball is approximately spherical in shape with a diameter of \({\rm{2}}{\rm{.5}}\,{\rm{cm}}\). The orbit or the eye socket is the bony cavity in the skull that holds the eyeball in it. The figure given below shows the structure of the human eye.

What are the Parts of the Eyes?

1. Cornea: It is a transparent protective membrane of the human eye present in front of the eye. It is the outer layer of the human eye. The cornea is the visible portion of the eye that can be seen from the outside. It helps to focus light inside the eye. The light gets refracted by the cornea before it enters the lens of the eye.

2. Aqueous Humour: It is a transparent watery fluid present behind the cornea. It maintains the pressure within the eye and helps in nourishing the eye lens. 

3. Crystalline lens: The eye lens is made of fibrous and jelly-like material. This part of the human eye is responsible for the formation of the image. It finely adjusts its focal length and focuses the light entering it onto the retina. The eye lens forms a real and inverted image of the object on the retina.

4. Ciliary muscles: The eye lens is attached to the ciliary muscles. It helps the eye lens to adjust its focal length. When we see any distant object, the ciliary muscles are relaxed so that the focal length of the lens increases, but when we see any object placed closed to us, the ciliary muscles contracts and the focal length of the eye lens decreases.

5. Iris: There is a muscular diaphragm called the iris between the aqueous humour and the lens. It controls the size of the pupil. If the amount of light entering the eye is large, then the iris contracts the pupil to reduce the size of the pupil. If the amount of light entering the eye is small, then the iris expands the pupil. Iris is the coloured part of our eye.

6. Pupil: It is the small opening present in front of the eye that controls the amount of light entering the eye. 

7. Vitreous humour or vitreous body: This is a transparent jelly-like substance present between the eye lens and retina. Its main function involves maintaining the shape of the eye and keeping it wet.

8. Retina: Retina is a delicate membrane that acts as a screen for image formation in the eye. It consists of a number of light-sensitive cells. These cells generate electrical signals when light falls on them.

9. Optic nerves: Optic nerves send the electrical signals generated by light-sensitive cells to the brain, and the brain interprets the signals and processes the information.

The point where the optic nerves are connected to the retina is called the blind spot. No light-sensitive cells are present at the blind spot, so we cannot see any image which forms on the blind spot of the eye.

Rods and Cones in the Human Eye

Rods and cones are the light-sensitive cells present on the retina of the human eye. Rods are rod-shaped cells that are sensitive to very low-intensity light. We can see objects present in a dark room due to the rod cells present in the retina of our eyes. These cells do not provide any information about the colour of the object. Owls can see in the dark night because they have a large number of rod cells in their retina. Cones are cone-shaped cells that are sensitive to bright light as well as normal intensity light. Cones can sense the colour of an object. These cells do not function in dim light.

Power of Accommodation

The ability of the eye lens to adjust its focal length is called accommodation. The ciliary muscles contract to decrease the focal length of the eye lens, and it relaxes to increase the focal length of the eye lens when we see nearby and distant objects, respectively. Beyond a certain limit, the focal length of the eye lens cannot be decreased, so the image of the object cannot be formed on the retina if the object is brought too close to the eye. The minimum distance of distinct vision of an object is equal to \({\rm{25}}\,{\rm{cm}}\).

Resolution of the Human Eye

The resolution of the human eye is \(576\) megapixels.

An image retains on the retina for about \(\frac{1}{{16}}\) of a second even after the removal of the object. This is called persistence of vision for a normal human eye. This property of vision is used in cinematography. Our brain takes \(13\) milliseconds to process an image.

Defects of Human Eye

The human eye slowly loses its power of accommodation with time. This makes people develop an eye defect and makes them uncomfortable to see objects clearly. There are four common types of eye defects.

1. Cataract: This is the condition in which the crystalline lens of people becomes cloudy and milky. It mostly happens with old-aged people. It causes a partial or complete loss of vision. Cataract can be cured by surgery.

2. Myopia or near-sightedness: A person suffering from myopia can clear see nearby objects but cannot see distant objects clearly. The far point of a myopic eye is nearer than infinity. In this case, the rays coming from a distant object, after refraction from the eye lens forms an image in front of the retina. This defect can be corrected by using a concave lens of suitable focal length.

3. Hypermetropia or far-sightedness: A person suffering from hypermetropia cannot see nearby objects clearly but can clearly see distant objects. The near point of a hypermetropia eye is farther away from the normal near point. In this case, the rays incident on the lens from the nearby object, after refraction forms an image behind the retina. This defect can be corrected by using a convex lens of suitable power.

4. Presbyopia: The gradual weakening of the eye’s ability to focus on nearby and distinct objects is called presbyopia. This defect develops with ageing. This defect can be corrected by using a bi-focal lens. The upper part of the bi-focal lens consists of a concave lens, and its lower part is a convex lens.

FAQs on Human Eye

Q.1. What are the parts of the human eye?
Ans: The human eye consists of the following parts:
1. Cornea
2. Pupil
3. Aqueous humour
4. Iris
5. Ciliary muscles
6. Eye lens
7. Vitreous humour
8. Retina
9. Optic nerves

Q.2. How does the human eye works?
Ans: The light rays coming from an object enter the human eye through the cornea and pupil of the eye. Most of the refraction occurs at the cornea of the eye. Now, these refracted rays enter the eye lens. The eye lens is a convex lens, so it converges the light rays such that they get focused on the retina. The real and inverted image of the object is obtained on the retina. The retina consists of a larger number of light-sensitive cells that generate electrical signals. These electrical signals are sent to the brain via optic nerves, and the brain interprets the signal and makes us perceive the object as it is.

Q.3. What colour attracts the human eye most?
Ans: The human eye is more sensitive to bright green colour because the eye is more sensitive to light at a wavelength of \(555\) nanometres. It stimulates rod and cone cells present on the retina of the eye.

Q.4. What is the resolution of the human eye in a megapixel?
Ans: The estimated resolution of the human eye is \(576\) megapixels.

Q.5. What fps can the human eye see?
Ans: The human eye can see and recognize between \(30\) to \(60\) frames per second. Frame rate which is expressed as frame per second \((fps)\) is the frequency at which the consecutive series of images can appear on a screen.

NCERT Class 10 PDF for Human Eye and Colourful World

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