This is a summary of the second part of Appendix A in Albert Einstein’s book [1]. By second part, I mean the part immediately after the derivation of the Lorentz transformation. If there is no relative motion between the coordinate systems with respect to the $ y$ and $ z$ axis of $ K$, then the $ Y$ and $ Z$ coordinates of the light in $ K$ are equal to the coordinates of the light in $ K’$, since there is no need to modify these coordinates: $ Y’ = Y$ and $ Z’ = Z$. Next, the […]