Question

what is: persistence of vision, focal length

Answer 1

persistence of vision is the time till which the image we see remains in our mind.
It is 1/16th of a second.
Focal length is length of focus point from origin.

Answer 2

1.The inability of human brain to perceive more than two images formed on the retina within an interval of 1/16th of a second is called Persistance of vision .
2.Focal length (shown in red) is the distance between the center of a convex lens or a concave mirror and the focal point of the lens or mirror — the point where parallel rays of light meet, or converge.