We were going along the Llangollen a few years back and noticed smoke coming from a unoccupied moored boat - a very nice new one. Arriving by the lock nearby someone had phoned the fire brigade while an old chap was staggerling across a field towards the boat clutching a massive extinguisher. To cut the story the fire brigade arrived, drove straight past those holding the field gate open, then returned and proceeded to chase a load of cows round the field with their siren and lights going. Finally they got to the boat and broke in to put the fire out. We later heard the whole back interior was destroyed and cost many thousands to fix. As part of the fix they put covers on the swish bulleye windows - one of which - it was said - was put in the wrong way round and so converged the light rather than dispersing it.
--- On Thu, 10/6/10, [email protected] <[email protected]> wrote: From: [email protected] <[email protected]> Subject: Re: [canals-list] Re: Roof lights and fires :-( To: [email protected] Date: Thursday, 10 June, 2010, 17:24 [email protected] wrote: > If there's any truth in that, then it's nothing to do with the basic optics > of how light passes through a plano-convex lens. A converging lens is a > converging lens in both directions. Further to what I wrote, I think I might have got to the bottom of it... Whilst the basic principle above is fundamentally true - a plano-convex lens converges light, whichever way round it is mounted - there is a difference in terms of the abberrations. No lens focuses light perfectly: there will be second-order effects such as spherical abberation (light rays passing near the edge of the lens aren't quite in focus with those passing near the centre) and chromatic aberration (different colours bend different amounts and therefore focus at different distances). In the case of a plano-convex lens, spherical aberration is more pronounced when the light passes from the flat side. It will still bend all the light inwards towards the centre, but the increased amount of aberration means that quite possibly it won't focus it sharply enough to start a fire. Whether this also explains Bruce's experience that it diffuses through the cabin better, I haven't worked out yet. See http://folk.uio.no/walmann/Publications/Master/node8.html for some diagrams. Martin L [Non-text portions of this message have been removed]
