George wrote:
>From http://en.wikipedia.org/wiki/Lens_(optics): > >"Lenses have the same focal length when light travels from the back to >the front as when light goes from the front to the back". But the same source also states: "Spherical aberration can be minimised by careful choice of the curvature of the surfaces for a particular application: for instance, a plano-convex lens which is used to focus a collimated beam produces a sharper focal spot when used with the convex side towards the beam source." Bullseyes are essentially plano-convex lenses. When placed convex side up, i.e. towards the beam (sun), a sharper focus is produced and so more likely to produce enough heat to burn something at the focal length. Placed convex side down the focus is less sharp and hence less dangerous. For those who wish to get into the maths of all this more detail can be found at http://folk.uio.no/walmann/Publications/Master/node8.html. The diagrams in this source suggest that the point of focus is mostly generated by the rays of light passing thought he centre of the lens, with the rays further out being focused progressively closer to the lens. This presumably explains why grinding or painting a spot in the centre of the bullseye has been found to reduce the risk of unintended combustion - it blocks the rays which focus at a single point. The ultimate convex-down deck prism is the 'lemon squeezer' type e.g. at http://www.boatdeckprism.com, but mind your head in a narrowboat cabin! David Mack
