If the radius of curvature of a spherical mirror is $30 \ cm$,what is its focal length (in $cm$)?

$(a)$ Water has a refractive index of $1.33$ and alcohol has a refractive index of $1.36$. Which of the two media is optically denser? Give a reason for your answer. Draw a ray diagram to show the path of a ray of light passing obliquely from water to alcohol.
$(b)$ The absolute refractive index of diamond is $2.42$ and the absolute refractive index of glass is $1.50$. Find the refractive index of diamond with respect to glass.

Define absolute refractive index. Absolute refractive indices of medium $A$ and medium $B$ are $n_{a}$ and $n_{b}$ respectively. What is the refractive index of medium $B$ with respect to medium $A$? How does the velocity of light vary with change in the optical density of the media?

State Snell's law of refraction.

If the focal length of a spherical mirror is $20 \ cm$,what is its radius of curvature (in $cm$)?

What is the point,where the rays parallel to the principal axis meet after reflection,called?