wave train

A "wave train" is a short "burst" or "envelope" of wave action that travels as a single unit. In other words, it is a succession of wave cycles of the same wavelength moving at the same speed.

Electromagentic waves are continuously emitted during the transition. GIven the transition takes a time interval of 10^-9 s and the speed of electromagentic radiation is 3 x 10^8 m/s, using the relationship: distance = speed x time.
The space (or length) at which the electromagentic wave spans = (3x10^8) x 10^-9 m = 0.3 m

其實即係講緊波既長度。