Ultrashort laser pulses have become an indispensable tool for atomic and molecular research. A new technology makes creating short infrared pulses easy and cheap.
In a marathon, everyone starts at roughly the same place at roughly the same time. But the faster runners will gradually increase their lead, and in the end, the distribution of runners on the street will be very broad. Something similar happens to a pulse of light sent through a medium. The pulse is a combination of different colours (or different wavelengths), and when they are sent through a medium like glass, they travel at slightly different speeds. This leads to a dispersion effect: the pulse becomes longer and longer.