Lenses
A transparent body bounded by one or two spherical surfaces is called a lens. A convex lens, thicker in the middle than at the edges, gathers parallel rays at one point on the principal optical axis, the principal focus F, so it is converging; a concave lens, thin in the middle, spreads rays out and its focus is virtual (the continuations of the rays meet). The distance from the lens centre to the focus is the focal length F, and a lens has one focus on each side. The optical power is D = 1/F, unit dioptre (1 dptr = 1 m⁻¹); for a converging lens F and D are positive, for a diverging lens negative. For a biconvex lens made of glass (n) in air with radii of curvature R₁ and R₂, D = (n – 1)(1/R₁ + 1/R₂). Smaller radii mean the lens bends light more strongly. Lenses are used in spectacles, magnifiers, cameras, microscopes and telescopes.
Observation: with a magnifier or a spectacle lens (with permission), form a sharp image of a distant object outside a window on a sheet of white paper. Measure the distance from the lens to the paper and find the approximate F and D. Never look at the Sun through a lens and never aim the light at eyes or flammable things!