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

Why does a light ray incident on a rectangular glass slab immersed in any medium emerge parallel to itself? Explain.

The velocity of light in a vacuum is $....$

What is the $SI$ unit of the power of a lens?

Draw ray diagrams showing the image formation by a concave mirror when an object is placed:
$(a)$ Between pole $(P)$ and focus $(F)$ of the mirror.
$(b)$ Between focus $(F)$ and centre of curvature $(C)$ of the mirror.
$(c)$ At centre of curvature $(C)$ of the mirror.
$(d)$ $A$ little beyond centre of curvature $(C)$ of the mirror.
$(e)$ At infinity.

What is the effect on the wavelength of light when it travels from a rarer to a denser medium?