Because of the postulates of relativity, it is impossible to reach the speed of light, so the Galilean velocity equations u^{\prime} = u – v and u = u^{\prime} + v cannot be used on light.
Note that velocity values are vectors, and that the speed u^{\prime}, is always with respect to S^{\prime} where S^{\prime} is in relative motion to S.
The Galilean equations are modified as follows to be applicable at light speed:
u^{\prime} = u-v becomes u^{\prime}= \frac {u-v}{1-\frac{uv}{c^2}}.
u=u^{\prime}+v becomes u=\frac{u^{\prime}+v}{1+\frac{u’v}{c^2}}.
These modified equations allow us to use Galileo’s velocity additions with the speed of light.