Eyelids and its functions
Nature has provided eyes with eyelids to prevent any object from entering the eye.
Eyelids also shut out light when not required.
Persistence of vision
The impression of an image does not vanish immediately from the retina.
It persists there for about 1/16th of a second.
So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.
The movies that we see are actually a number of separate pictures in proper sequence.
They are made to move across the eye usually at the rate of 24 pictures per second (faster than 16 per second). So, we see a moving picture.
Range of vision of a Normal Human Eye
The farthest point from the eye at which an object can be seen clearly is known as the far point.
The far point of a normal human eye is infinity .
The nearest point upto which the eye can see objects clearly without any strain, is known as the near point.
The near point of a normal human eye is 25 centimetres from the eye.