Des Schofield, of King's High School, asks :-

If a torch was shone around a closed ring of mirrors, what would happen when the torch was turned off?

Clive Rowe, a member of the Canterbury Ring Laser Group, responded.

It so happens our group performed this experiment a few months ago. Rather than a torch we used a ring laser, in which the helium-neon laser beam is reflected around a square path by four mirrors at the corners of the square. The side of the square is one metre.

Light becomes less intense when the light is absorbed by the mirrors or scattered out of the beam by imperfections so the intensity decreases with every reflection or scattering.

We were able to switch the laser off very suddenly, in less than a tenth of a millionth of a second. We found that the light being reflected around the closed path faded away within about ten millionths of a second.

The fading time depends on how shiny the mirrors are. Ours are extremely good reflectors, some of the best in the world. Each mirror reflects over 99.997 percent of the laser light incident on it.

Light travels at about 300,000 km every second, that is 300 metres every millionth of a second so in 10 millionths of a second each photon of light travelled about three kilometres, ie several thousand times around the ring before being absorbed or scattered out of the beam.