This is the most useful infomration I've read in ages and I feel it will make me more popular by far. Hooray for NewScientist!
Sparklers appear to draw lines in the air because of the phenomenon known as visual persistence. The human eye does not react instantly when its view changes, but keeps the old image around for a few milliseconds. This is what enables us to perceive films or television images as moving pictures when they are in fact a sequence of still images. The persistence of the eye causes each image to merge into its successor, creating the illusion of movement.
If the changing image contains very bright objects against a dark background - such as a sparkler at night - the persistence lasts longer, so the light from quite a long period of time can be added together to appear as a single streak.
There are numerous gadgets that exploit this effect by using strips of fast-moving LEDs to apparently create writing in the air. Persistence can also be seen in the coloured spots left in your vision after a camera's flash has gone off.
The sparks from the sparkler are produced by burning flecks of a metal such as magnesium or aluminium flung off from the from the firework. Initially only their outer layer of metal burns, but after the fleck has burnt down to a critical size the core becomes so hot that it explodes. The sub-flecks from the explosion then burn out quickly and brightly in a distinctive star.
Posted by bluprnt at March 24, 2006 04:12 PM