Question

how will you find the concave or convex lens without touching it

Answer 1

To differentiate between convex lens and concave lens simply use few LASER lights and arrange them in parallel taking some distance and place the lens one by one firstly if the LASER lights converge at a point in the direction of the light then that lens is convex and if the light diverge after passing through the lens then the lens used is concave lens.


Or


Place it on a paper (obviously written) if the image of paper gets magnified >1 (i.e larger than original) then its convex lens else concave(magnification<1)...

Answer 2

we can differentiate between convex and concave lens by looking through it as concave lens always forms virtual and erect image but convex lens will form real and inverted image in most of cases..