As we know that
[tex]\mu = \frac{speed of light in air}{speed of light in medium}[/tex]
now for the first part we know that
refractive index of vacuum with respect to glass is 2/3
speed of light in glass is [tex]2 * 10^8 m/s[/tex]
now we will have
[tex]\frac{2}{3} = \frac{2* 10^8}{c}[/tex]
[tex]c = 3 * 10^8 m/s[/tex]
Part b)
Now similarly we can say
[tex]\mu = \frac{speed of light in air}{speed of light in medium}[/tex]
here refractive index of water with respect to vacuum is 4/3
[tex]\frac{4}{3} = \frac{3 * 10^8}{v}[/tex]
now we will have
[tex]v = 2.25 * 10^8 m/s[/tex]
