At the point where the optic nerve enters the eyes, there is no retina, and consequently, this area is not light-sensitive. This point, which is just below the center of the back of the eye, is called blind spot.

You can prove the existence of this blind spot in the following manner. Note capital T and the capital O on this page, below this article. Close your left eye, fix your gaze on the T. Now move at an eye level towards the screen and away from the screen, until you find the point where the O completely disappears. At this point the O is focused by the lens of the eye exactly on your blind spot. Hence, you cannot see the O.

      T                                                             O