What is the centre of a curved mirror called?

Where does an object and its image coincide in a concave mirror?

What happens when a light ray incident normally on the surface of a glass slab enters from air into the glass?

Light enters from air into diamond,which has a refractive index of $2.42$. Calculate the speed of light in diamond. The speed of light in air is $3.0 \times 10^{8} \text{ m s}^{-1}$.

Define linear magnification produced by a spherical mirror. Also,give its relation.

$(a)$ For the same angle of incidence $45^{\circ}$,the angle of refraction in two transparent media $I$ and $II$ is $20^{\circ}$ and $30^{\circ}$ respectively. Out of $I$ and $II$,which medium is optically denser and why?
$(b)$ Light enters from air to diamond,which has a refractive index of $2.42$. Calculate the speed of light in diamond,if the speed of light in air is $3.00 \times 10^{8} \text{ m s}^{-1}$.