What happens to the image formed by a convex lens if its lower part is blackened?

Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

How will you identify a concave lens by touching it?

Find the focal length of a lens of power $-2.0 \, D$. What type of lens is this?

An object $3.0 \text{ cm}$ high is placed perpendicular to the principal axis of a concave lens of focal length $7.5 \text{ cm}$. The image is formed at a distance of $5.0 \text{ cm}$ from the lens. Calculate $(i)$ distance at which object is placed,and $(ii)$ size and nature of image formed.

An object $5\, cm$ high is kept $25\, cm$ away from a converging lens (convex lens) of focal length $10\, cm$. What is the nature,position,and size of the image?