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Human Eye

Human Eye:

The Human Eye is a sense organ that reacts to light and allows vision. The human eye can differentiate between about 10 million colours and is possibly capable of detecting a single photon. The eye is part of the sensory nervous system. It consists of two types of cells – Rod cells & Cone cells.

Structure of Human Eye

  • The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of an anterior (front) segment and the posterior (back) segment.
  • There are two eyes, situated on the left and the right of the face. They sit in two bony cavities called the orbits, which are present in the skull.
  • Six extraocular muscles attach directly to the eyes to assist with movement. The front visible part of the eye is made up of the whitish sclera, a coloured iris, and the pupil.
  • The front part is also called the anterior segment of the eye. The cornea is transparent and more curved, & is linked to retina and the outer white shell called the sclera.
  • The cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black. 

Do you know How we are able to see things around us?

It often comes in the exams.  The answer to this is: The Human eye works on the refraction of light through a natural convex lens made up of transparent living material and enables us to see things around us. 

The Various Parts of Eye and their Functions:

  • Cornea :It is a thin membrane through which light enters.
  • Eyeball : It is a convex lens, approximately spherical in shape, with a diameter of about 2.3 cm. It can alter its curvature with help of ciliary muscles.
  • Iris :It is a dark muscular diaphragm that controls the size of pupil. It helps in accommodation of light by changing the size of the pupil.
  • Pupil : It regulates and control the amount of light entering the eye.
  • Ciliary muscles:It helps to change the curvature of eye lens, so that we can see the object clearly placed at different positions.
  • Retina :Thin membrane with large no. of light sensitive cells.

There are two types of photoreceptors in the human retina, rods and cones. Rods are responsible for vision at low light levels. Cones are active at higher light levels, are capable of color vision.

How is image formed at retina ?

When image is formed at retina, light sensitive cells get activated and generate electrical signal. These signals are sent to brain via optic nerve. Brain analyses these signals after which we perceive object as they are.

In the Human Eye, there are three common defects of vision. They are:

1) Myopia (Short-sightedness or Near-sightedness)

2) Hypermetropia (Long-sightedness or Far-sightedness)

3) Presbyopia


 Myopia – This is also called as the short-sightedness. A person with this eye defect can only see the nearby objects clearly compared to distant objects. This condition can be corrected using a concave lens.

Hypermetropia – This is also called as the farsightedness. A person with this eye defect can only see the distant objects clearly compared to near objects. This condition can be corrected using a convex lens.

Presbyopia – This is an age-related condition caused due to the weakening of ciliary muscles, hardening of the lens, and reduced lens flexibility. A person with this defect usually finds difficulties to focus on nearby objects unable to read or write.

Cataract – This is an age-related condition caused due to the loss of transparency of the lens by erosion of lens proteins. It usually results in blurry vision, cloudy lenses and cannot be corrected by any type of lens but we can replace the old lens with an artificial lens.

Let’s have a look at some questions:

Q1) Myopia is same as ? 

a) Near sightedness         

b) Astigmatism

c) Presbyopia                    

d) Long sightedness

Ans: (a)

Q2) Which part of the Human Eye changes it size based on the amount of Light? 

a) Iris                   

b) Pupil

c) Retina

d) Cornea

Ans: (b)

Q3) Which lens is used to correct Hypermetropia? 

a) Concave

b) Bifocal

c) Convex

d) cannot be corrected by any type of lens

Ans: (c) 

Do you know the main structures in the anatomy of a Human Eye?

The SCLERA, or white part of the eye, protects the eyeball. The PUPIL, or black dot at the centre of the eye, is an opening through which light can enter the eye. The IRIS, or coloured part of the eye, surrounds the pupil.

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