IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Blind Spot of Eye

## Objective

To feel for the blind spot of eye.

## Introduction

There are millions of photo sensors at the surface of the retina. These sensors are at the tips of nerves which take the voltage pulse generated to the brain. All these nerves leave the eyeball from a small area of retina say about $$2\,\mathrm{mm^2}$$. This particular part of the retina has no sensors in it and is called the blind spot of the eye. If the image of a small object falls on this part of the retina, the object will not be seen.

## Apparatus

A paper (preferably ruled one), a pen.

## Procedure

1. On a horizontal line of the paper, make two marks using the pen. Each mark may be of 2-3 mm size and the separation between the marks may be say 10 cm to start with. Write the distance in the table.
2. Keep the paper at a distance of about 40 cm from the eyes. Bring the left spot in front of the right eye. Close the left eye. Focus on the left spot.
3. Very slowly bring the paper closer to yourself. All the time keep looking at the left spot. At a certain distance from the eye, you will find that the right spot has become invisible.
4. Measure the distance of the paper from the eye and write in the table.
5. Repeat with two more separations between the spots, say 12 cm and 16 cm.
6. Close your right eye. Bring the left spot in front of your left eye. Try changing the distance of the paper from the eye. Are you able to find a position of the paper where the right spot becomes in visible? Explain.

## Discussion

When you look at a particular spot, the position and focal length of the eye-lens gets fixed. Now different objects create images at different portions of the retina. The object that gives image at the blind spot is not visible. If you are looking at the left spot with left eye, the right spot will make image on the retina, left to the center. That is on the side opposite to the nose or outer side. But if you are looking at the left spot using the right eye, the other spot makes image on the inner side. Using your observations, which case the spot becomes invisible, you can determine whether the blind spot is on the inner side or the outer side of the eye.