Physics

The Doppler effect is the change in a wave's observed frequency caused by relative motion between source and observer. How it works for sound, light and radar.

A half-life is the time it takes for half of a quantity to decay. What it means in radioactivity, how it dates objects, and where else it applies.

Nuclear fusion is the joining of light atomic nuclei into a heavier one, releasing energy. How it powers the Sun and why it is hard to harness on Earth.