My son was playing with sparklers on Bonfire Night, and wanted to know why he could draw shapes in the air with them. Then he wanted to know why each 鈥渟parkle鈥� that shot off the wire he was holding consisted of a distinctive line ending in a star. I couldn鈥檛 answer either question. Can anyone help?
鈥� 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 鈥� such as a sparkler at night 鈥� the persistence lasts longer, so the light from quite a long period of time can be added together to appear as a single streak.
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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鈥檚 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.
Alec Cawley, Newbury, Berkshire, UK