BasicRelativity
Shift the slider f
The light clock is restricted to one vertical ping-pong of light at rest and one observer in motion.
According to the principle of relativity switching the roles must give the same results. By sliding f you can do here more than that: Both may switch the roles partly, move a little bit such that the relative velocity (v_R+v_B)/c = v/c remains invariant. We may detect that the time indicated by t [nuts] does vary despite ct does not.