Which symbol is used for the refractive index?

When an object is placed at the center of curvature $(C)$ in front of a concave mirror,what kind of image is formed?

The angle between an incident ray and the reflected ray is $70^{\circ}$ as shown in the figure. What is the value of $\theta$?

The image formed by a lens is always erect and diminished. What is the nature of the lens?

When does Snell's law fail?

As the velocity of light increases,the refractive index of the medium decreases. Light enters from air to water having a refractive index of $4/3$. Find the speed of light in water. The speed of light in vacuum is $3 \times 10^{8} \text{ m s}^{-1}$.