sir what will we take the value of speed of light if we apply the formula for mass expansion if an object is moving in water?

the speed of light changes as it moves from one medium to another

assuming that speed of light in air is 3*10⁸ m/s

the speed of light in water is taken as 2.25 x 10^8 m/s as it is 3/4 of speed of light in vaccum !

Normally for such questions , the value will be given , but if they aren't given then keep it as a constant as it will get cancelled out later .

Hope that helps

Please approve !

In special relativity, the speed of light in ANY formula, in ANY medium(including water), should always be taken as the speed of light in vacuum, which is a CONSTANT.