**Given; **

Refractive index of (R.I) water w.r.t vacuum = \(\frac43\)

Refractive index (R.I) of vacuum w.r.t glass = \(\frac23\)

Refractive index (R.I) of glass to vacuum will be = \(\frac32\)

Refractive index of glass to water = \(\frac{R.I\,of\,glass}{R.I\,of\,water}= \frac{\frac32}{\frac43}\)

⇒ Refractive index of glass w.r.t water = \(\frac{3\times3}{4\times2}= \frac98\)

⇒ \(\frac{speed\,of\,light\,in\,water}{speed\,of\,light\,in\,glass}= \frac98\)

⇒ \(\frac{speed\,of\,light\,in\,water}{2\times10^8}= \frac98\)

⇒ Speed of light in water = \(\frac{9\times2\times10^8}{8}\)

⇒ Speed of light in water = \(\frac{9\times10^8}{4}\) = 2.25 x 10^{8}

Thus, speed of light in water = 2.25 × 10^{8}

Refractive index of water w.r.t vacuum = vμw** = **\(\frac43\)

⇒ \(\frac{speed\,of\,light\,in\,vacuum}{speed\,of\,light\,in\,water}= \frac43\)

⇒ \(\frac{speed\,of\,light\,in\,vacuum}{2.25\times10^8}= \frac43\)

⇒ Speed of light in water = \(\frac{4\times2.25\times10^8}{3}\)

⇒ Speed of light in water = \(\frac{9\times10^8}{3}\)

Thus, speed of light in vacuum is 3 × 10^{8} m/s.