What type of image is always formed by a concave lens?

If the absolute refractive indices of water, benzene, and sapphire are $1.33$, $1.50$, and $1.77$ respectively, then which medium has the maximum relative refractive index?

If the radius of curvature of a concave mirror is $36\, cm$,what is its focal length (in $, cm$)?

What is the absolute refractive index of water?

When an object is placed at $2F$ in front of a convex lens,where will its image be formed?

Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of $15\, cm$ from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?