The objective: To find and understand the blind spot of your eye with a simple experiment.
A stiff piece of paper of card approximately 8 x 10 cm in dimension.
A metre ruler.
Draw a dot and a cross on the paper at the same level, a few inches apart from each other.
Hold the paper or card at arm’s length at eye level so that the dot is on the right side.
Keep your right eye closed and look directly at the dot with your left eye. In the beginning you will also be able to see the cross. Bring the card closer to your face, focussing on the dot but also seeing the cross. As you bring it closer, at one point the cross will disappear then reappear as you bring it further closer.
Next, draw a straight line through the cross and dot and repeat the experiment, focussing on the dot with your left eye. As you bring the card closer, the dot will disappear but the line will still appear continuous
We see objects using our eye's retina. It receives incoming light and sends signals to the brain which interprets the signals and allows you to see. However there is a spot on the retina which does not give you visual information.
The retina does not have any light receptors at this spot and if light falls on this spot, we cannot see it, When you bring the card closer and closer, at one distance the light from the cross will fall on the blind spot and it will disappear. Same thing happens when you repeat the experiment with the dot.
Later when you draw a straight line through the dot and bring the card closer, the light from the dot falls on the blind spot and it disappears. However you still see the line as continuous as the brain automatically fills in the missing part based on the remaining part of the line. This is why we never notice the effect of the blind spot in day to day life.