[email protected] wrote: > [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 >
Well found diagram, I was trying to find one myself - I was sure you were correct BTW. Maybe the best answer is to frost the flat side with some glasspaper, it will reduce the light input a tad, but will kill the focus. Ron Jones Process Safety & Development Specialist Don't repeat history, unreported chemical lab/plant near misses at http://www.crhf.org.uk Only two things are certain: The universe and human stupidity; and I'm not certain about the universe. ~ Albert Einstein
