Question

how will you distinguish between a concave and convex lens without touching it

Answer 1

A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
A concave lens also forms erect, virtual and smaller image than the object.

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.