Question

by just looking at the spectacles how can a person judge whether the spectacles is for long sightedness or short sightedness.

Answer 1

if the lens is thick at periphery and thin at centre ,
 then it is a concave lens . 
it is used for correcting myopia (short sightedness )
 if lens is thin at periphery and thick at centre ,
then it is a convex lens .
it is used for correcting hypermetropia/hyperopia ( long sightedness )

Answer 2

Answer:

If the lens is convex =>Person is longsighted

If the lens is concave =>Person is shortsighted