Question

how can we differentiate between convex and concave lens without touching them

Answer 1

How will you distinguish between a convex and a concave lens without touching them? We will keep the lens close to some page of a book and see the print of the book through it. If the letters of the book appear enlarged, it is a convex lens and if they appear diminished, it is concave lens..
I hope this is useful for you

Answer 2

We can differentiate between a convex and concave lens by seeing the image.

• On keeping the lens near a printed page, if letters appear magnified, the lens is convex and if the letters appear diminished, the lens is concave.
• On seeing a distant object through the lens, if its inverted image is seen, the lens is convex and if an upright image is seen, the lens is concave.