[email protected] wrote: > Trevor Sheppard > <[email protected]> wrote:
[regarding focusing of light by bullseye rooflights] > I've heard tell that this problem can occur only if the bullseye is > incorrectly installed, which is a common mistake. > AIUI, the bullseye should be installed with the curved (convex) side > *down*. This lights the interior better, by dispersing the light. If > the bullseye is installed the other way up, it concentrates the light > (and heat) on a point in the interior. I've heard that too, but I'm unconvinced. My memory from studying physics (quite as long time ago, admittedly) is that the paths taken by light rays passing through a lens are 'reversible'; in other words if a lens converges light passing through it in one direction, then it will also converge light passing through it in the opposite direction. A plano-convex lens (which is what a bullseye approximates to) will therefore concentrate the light (and heat) to a point in the interior of the boat, whichever way up it is installed. The focal point might well end up a different distance from the roof of the boat (anyone care to do the sums?) and at low sun angles there might be a difference in the amount of light that it 'catches'), but the basic principle will remain the same. Or have I remembered it wrongly? Martin L
