This question is based on refraction of light between air and water.
In the first part light is at air medium. At the second instance it goes from air to water which is refraction.
The speed of light with velocity `c` , frequency` f` and wave length `lambda` can be given as;
`c = flambda`
`lambda = c/f`
The speed of light in air is `3xx10^8 m/s`
`lambda = (3xx10^8)/(4.74xx10^14)`
`lambda = 6.33xx10^(-7)m`
The wave length of the laser line is `6.33xx10^-7m`
When the lase beam goes in to water refraction occurs. Here the velocity of lase in the two phases are different. But the important thing is wave length is a constant in both mediums.
Using Snell's law we can get the speed of laser in water.
`V_1/V_2 = n_2/n_1`
The refractive index of water is 1.33 and air is 1.00
`(3xx10^8)/V_2 = 1.33/1.00`
`V_2 = 2.256xx10^8m/s`
So velocity of laser line in water is `2.256xx10^8m/s.`