Asked by: Jessica Connor, Brighton
It may sound like an urban myth, but it can and does happen. Fishbowls, jam-jars and even glass door-knobs have been implicated in focusing the sun’s rays sufficiently to cause smouldering, followed by a full-scale blaze.
Earlier this year, London Fire Brigade said that 125 fires have been triggered by the sun’s rays over the last five years – and warned that the risk exists during the winter as well as summer. That’s because the sun’s rays bathe the Earth in a constant flow of thermal energy spread over each square metre. While this is too dilute to ignite paper, wood or other combustible substances, if the rays are focused, the flow of energy becomes concentrated enough to exceed the threshold for combustion. Magnifying glasses do this very effectively, refracting the rays and bringing them to a tight focus. But even fragments of glass can have some focusing effect – with potentially disastrous consequences.
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