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

Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in Figure?

The rays travelling parallel to the principal axis after reflecting from the concave mirror pass or appear to pass through the focus.

$(a)$ 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.

Which of the following can produce a magnification of $+0.5$?

In which medium is the speed of light the highest?