In vacuum, the wave has a wavelength of
[tex]\lambda=600 nm=6\cdot 10^{-7}m[/tex]
and it travels at speed of light
[tex]c=3 \cdot 10^8 m/s[/tex]
So its frequency is
[tex]f= \frac{c}{\lambda}= \frac{3 \cdot 10^8 m/s}{6 \cdot 10^{-7}m}=5.0 \cdot 10^{14}Hz [/tex]
The frequency of an electromagnetic wave does not change when moving from a medium to another, and since the new speed of the wave in the liquid is
[tex]v=2.00 \cdot 10^8 m/s[/tex]
we can find its new wavelength when moving in the liquid:
[tex]\lambda'= \frac{v}{f}= \frac{2.00 \cdot 10^8 m/s}{5.0 \cdot 10^{14}Hz}=4.0 \cdot 10^{-7} m = 400 nm [/tex]
