Night vision;The big picture;Features amp; Arts

5th November 1999 at 00:00
The fifth of November is remembered for gunpowder, treason and a plot - a botched attempt to blow up the Houses of Parliament. But for most fans of Mr Fawkes's finest hour, the big attraction of tonight's celebrations will be fireworks.

Watching these spectacular displays might involve standing in a boggy field, and they may be over in a few minutes, but they still have the power to captivate us.

Lights in the sky, whether sunsets, eclipses, the northern lights or meteor showers, have always entranced us. But fireworks give a particularly intense kind of visual pleasure. As they emerge brightly from the dark night sky, they stimulate the rods and cones on the eye's retina that pick up light - especially the cones, which are particularly sensitive to colour and bright light.

When a rocket explodes, the showers of light trace arcs on to the retina's surface - close your eyes and you will see the pattern for a few seconds afterwards. In a flash, a message goes to the visual cortex of your brain, which interprets the light and movement making you think - wow!

The chemistry of fireworks began with the invention of gunpowder, probably in China around the seventh century ad. This mixture of charcoal, sulphur and potassium nitrate is still the main ingredient of fireworks. Like television, early displays were in black and white - colour and brilliance came with the addition of sodium, aluminium and magnesium.

Another kind of chemistry is at work in photography as light reacts with tiny crystals of silver suspended in gelatin-coated film to form an image on the negative. By standing the camera on a tripod, leaving the shutter open for half a second and focusing only after the firework explodes, the photographer has achieved this psychedelic effect. Proof that, when it wants to, the camera can indeed lie.

Picture by Akira Inoue.

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