Let us consider two inertial frames $S$ and $S'$ in which frame $S'$ is moving with constant velocity $v$ along the positive x-axis direction
relative to the frame $S$. Let $t$ and $t'$ be the time recorded in two frames. Let the origin $O$ and $O'$ of the two reference systems coincide at $t=t'=0$.
Now suppose, a source of light is situated at the origin $O$ in the frame $S$, from which a wavefront of light is emitted at time $t=0$. When
the light reaches point $P$, the time required by a light signal in travelling the distance OP in the Frame $S$ is
$ t=\frac{OP}{c}$
$ t=\frac{\left (x^{2}+y^{2}+z^{2} \right )}{c}$
$ x^{2}+y^{2}+z^{2}=c^{2}t^{2}\qquad (1)$
The equation $(1)$ represents the equation of wavefront in frame $S$. According to the special theory of relativity, the velocity of light will be $c$ in the second frame $S'$. Hence in frame $S'$ the time required by the light signal in travelling the distance $O'P$ is given by
$ t'=\frac{O'P}{c}$
$ x'^{2}+y'^{2}+z'^{2}=c^{2}t^{2}\qquad (2)$
According to the Galilean transformation equation:
Now substitute these values in equation $(2)$ then we get
$ (x-vt)^{2}+y^{2}+z^{2}=c^{2}t^{2}$
$ x^{2}+v^{2}t^{2}-2xvt+y^{2}+z^{2}-c^{2}t^{2}=0$
The above equation is certainly not same as the equation $(1)$ because it contains an extra term $(-2xvt+v^{2}t^{2})$. Thus the Galilean transformation fails.
Further $t=t'$ because $\left( t=\frac{OP}{c} \: and \: t'=\frac{O'P}{c} \right)$ which does not agree with Galilean transformation equations.
The extra term $(-2xvt+v^{2}t^{2})$ indicates that transformations in $x$ and $t$ should be modified so that this extra term is cancelled. So modification in transformation
$ x'=\alpha (x-vt) \quad for \: x'=0,\: x=vt$
$ t'=\alpha (t+fx)$
Where $α$, $α'$ and $f$ are constant to be determined for Galilean Transformations $α= α'=1$ and $f=0$. Now substituting these modified values in equation $(2)$ so
These equations are called Lorentz Transformations because they were first obtained by Dutch Physicist H. Lorentz.
The above transformation equation shows that frame $S'$ is moving in positive x-direction with velocity $v$ relative to the frame $S$. But if we
say that frame $S$ is moving with $v$ velocity relative to frame $S'$ along negative x-direction then the transformation is: