What is the centre of a curved mirror called?

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

On what factor does the lateral shift produced by a glass slab depend?

$A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from the lens. Where should the needle be placed in front of the convex lens so that this image is of the same size as the object? Also, find the power of the lens.

Both a spherical mirror and a thin spherical lens have a focal length of $-15 \; cm$. What type of mirror and lens are they?

How many times is the velocity of light in a vacuum compared to the velocity of light in water?