This morning's visit to the MSc optics lab with Peter Torok revealed this fantastic model for visualising the effect of a lense on light. Regard the lens positioned perpendicular to surface, to the left of the model. The highest peak is the focal point. These days, students usually see this rendered in 3-d on computer.
above - move optics lab
above - a model of the surface of light waves made in 1876, based on Fresnel's work in the 1820's on crystal optics. The surface represents a wave front of light radiating from a point in a crystal: Two shells touch each other in four places. (displayed in the Science Museum).
below - my shadow, south ken wall