Is power negative or positive for a concave lens?

$A$ thin lens of focal length $f$ and its aperture has a diameter $d$. It forms an image of intensity $I$. Now, the central part of the aperture up to diameter $(d/2)$ is blocked by an opaque paper. The focal length and image intensity would change to:

$A$ thin lens made of glass of refractive index $\mu_g = 1.5$ has a focal length of $12 \ cm$ in air. If it is immersed in water of refractive index $\mu_w = 4/3$,what will be its new focal length in $cm$?

$A$ convex lens is immersed in water. How will its power $P$ change?

An unsymmetrical double convex thin lens forms the image of a point object on its axis. Will the position of the image change if the lens is reversed?

The radius of curvature of the convex surface of a thin plano-convex lens is $15 \ cm$ and the refractive index of its material is $1.6$. The power of the lens is