[email protected] wrote: >[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.
>Or have I remembered it wrongly? I think you may have. Draw a vertical cross-section of the bullseye. Orient it so the convex side is up. Now draw a sun's ray hitting it vertically from above. My memory of physics is that the ray will be redirected from the vertical when it hits the bullseye, in the direction that brings its angle with the surface of the bullseye at the point of intersection nearer to the perpendicular to that surface. They will be further deflected when they leave the bullseye Assuming that, then draw some other angles, which will show that the redirected rows get concentrated (i.e. focused). Now orient the bullseye with the flat side up, and draw some more rays. These will pass straight through the flat surface, but will be bent away from the centre of the bullseye when they leave it. I think this means that they can burn in the first case, but can't in the second. But I don't have my high school optics text ... Adrian
