Physics – Structure of the Human Eye – CBSE Class 8


We learnt that objects are visible because of the light getting reflected from their surfaces. This reflected light enters our eyes and we see that object. What happens inside the human eye when light rays enter it? What is it that helps us see the things around us? The human eye is roughly spherical in shape. What we’re going to do in this video is understand the different parts of the eye in short… not in extreme detail. Let’s begin! The outer layer of the eye is called the CORNEA. It is white in colour and transparent. Transparent? Yes, it means that it allows light to pass through it! It acts like a tough front wall of the eye. What is its function? It protects the interior of the eye from accidents and also does not allow foreign particles to enter the eye. Okay, so this was all about the outer layer of the eye. Let’s now talk about the things present INSIDE. Behind the cornea there is a dark muscular structure called the IRIS. Iris determines the colour of the eye. For instance when we say that a person has blue eyes, it means that the colour of the iris of person’s eye is blue. Now there is a small opening at the centre of the iris for the light to enter. Do you know what it’s called? It is called the PUPIL. The Iris actually acts like a shutter of a camera. It controls the amount of light entering the eye by enlarging or shrinking and thereby adjusting the size of the pupil. For example, when you focus the light from the torch directly into a person’s eye the iris shrinks. This is because the intensity of light is too much for the eyes to bear! The shrinking iris stops the unwanted light from entering through the pupil. And when it’s dark or when there’s less light, it expands allowing more light to enter through the pupil. Ok, what happens next? The light that enters through the pupil falls on the LENS kept behind This lens is a CONVEX LENS. What is a convex lens? Convex lens is a lens which is thicker at the centre than the edges. It is a CONVERGING LENS. That is, it focuses all the light at one point. In a normal regular eye this point is somewhere here, on this inner boundary called RETINA. Why do i say a normal eye? Well, in a defective eye, this point of focus may lie either in front of the retina or behind the retina. We will talk about this defect of eye in our future videos but first let’s go back to our normal eye where light is focussed ON the retina. Retina is at the back of the eye and marks the inner layer of the eyeball. Retina contains lots of NERVE CELLS that sense light. The nerve cells are of two kinds namely CONES which are sensitive to bright light and RODS which are sensitive to dim light. The message or sensation is then transferred to the brain along the optic nerve. Ok, let’s quickly recap what we learned about the human eye. Human eye has a roughly spherical shape. The outer part of an eye called CORNEA is a thick white transparent wall, which protects the eye from accidents and keeps the foreign particles from entering inside. Behind the cornea, the colourful part of the eye called IRIS is present. It has a small opening at the centre called PUPIL. Iris controls the light entering through the pupil by contracting and expanding. Behind the pupil there is a converging lens which focuses the light on retina. Retina consists of two types of nerve cells namely cones and rods. They transfer the message through the optic nerve. That’s a lot of information to digest! So we will stop here and continue to learn more about the eye in the next lesson.

89 comments

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  5. It helped me a lot tomorrow is my final exam and I understood it very clearly and I would like to give you the whole credit to you for this mam thank you…. I would request you to please make videos like this. It would be much helpful for others may God bless you…πŸ™πŸ™πŸ™

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