Einstein's Light Clock
On page 272 of the book Einstein's Clocks, Poincaré's Maps by Peter Galison, I found this description of time dilation beautiful in its simplicity:
Figure 5.12 Einstein's Light Clock (1913). (a) In this simplest of all explanations of the dilation of time, Einstein imagined two parallel mirrors with a pulse of light reflecting between them, each traversal constituting a "tick." If a clock like this flew by an observer "at rest," the observer at rest would see the pulse following a sawtooth pattern. (b) Each angled traversal would follow a longer (diagonal) path than in the straight up-and-down path of a similar clock in the rest frame. Since light travels at the same speed in every frame of reference, Einstein concluded that a tick in the mirror frame would be measured as taking longer than a tick in the rest frame. Therefore, the rest observer must conclude, time runs slow in the moving frame of reference.