Why does time slow down for objects moving near the speed of light? Explain using the light‑clock idea. (NCERT Class 11, Relativity – Conceptual Important)

Answer: Everyone measures light at the same speed c. In a light‑clock, a photon bounces between two mirrors. At rest, its path is straight up‑down. If the clock moves sideways fast, an outside observer sees the photon follow a longer diagonal path, yet still at speed c. Since speed = distance/time and c is fixed, the longer distance means more time per tick. Therefore moving clocks tick slower—time dilates—so that c stays constant for all observers.