Question

how can i understand,human's eye have power?

Answer 1

When the eye sees an object at a long distance, its muscles are relaxed and the eye lens focuses the light rays entering the pupil, onto retina. In a healthy person it is about 2 cm away from eye lens.  Eyelens is a convex lens and it converges the image on to the retina.
Power of eye lens = 1 / focal length = + 1/ (2 cm)  = + 1/(0.02 meter) =  + 50 Dioptre

The ability to converge parallel light rays to form an image is the power.

A healthy human eye can normally see (in adults) CLEARLY from 25 cm onwards up to long distance, with out any distortion and distinctly. It will be clear when one tries to read a newspaper placed around 25 cm from the eye, whether he/she has a defective power.