• Question: why can light make a fire through glass and yet it wont set fire to a sofa through a usual lounge window?

    Asked by to Matthew on 11 Mar 2014. This question was also asked by .
    • Photo: Matthew Malek

      Matthew Malek answered on 11 Mar 2014:

      It all comes down to how the rays of light are focused. A regular pane of window glass allows light to pass straight though, but doesn’t focus them at all.

      In contrast, a lens is a piece of glass that bends the light rays. It has a “focal point”, which is where all the light that passes though ends up. By concentrating the light on this focal point, the amount of heat energy on anything placed there is increased, possibly causing a fire.

      You can unintentionally create a lens with a piece of glass or a mirror that focuses light unintentionally. That’s what happened in London this past summer, when the “walkie-talkie building” set fires, melted pavement… and even ignited someone’s Jaguar!

      (If you haven’t already heard about this, see here: http://www.bbc.co.uk/news/uk-england-london-23948811)

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