As we know that refractive index of any medium with respect to vacuum is defined as
[tex]\mu = \frac{speed of light in air}{speed of light in medium}[/tex]
now its given that refractive index of vacuum with respect to glass is 2/3
[tex]\frac{2}{3} = \frac{speed of light in glass}{speed of light in air}[/tex]
[tex]\frac{2}{3} = \frac{2 * 10^8}{c}[/tex]
[tex]c = 3 * 10^8 m/s[/tex]
Part (ii)
Now similarly we have
refractive index of water with respect to vacuum is 4/3
[tex]\mu = \frac{speed of light in air}{speed of light in water}[/tex]
[tex]\frac{4}{3} = \frac{3*10^8}{v}[/tex]
now we will have
[tex] v = 2.25 * 10^8 m/s[/tex]
