Lenses
A lens is a transparent body bounded on two sides, or one side, by a spherical surface (usually glass). If it is thicker in the middle than at the edges it is a convex lens, if thinner in the middle a concave lens. The straight line through the centres of the spherical surfaces is the principal optical axis. Rays parallel to the axis meet at a point — the focus (F) — after a convex lens, so it is called a converging lens; rays after a concave lens spread out, and their extensions meet at the (virtual) focus, so it is a diverging lens. The distance from the lens’s optical centre to the focus is the focal length F. The reciprocal of the focal length is the optical power: D = 1 : F; its unit is the dioptre (D); if F = 1 m, D = 1 dioptre. For a concave lens D is negative. A convex lens gives an upright, enlarged, virtual image when the object is nearer than F (a magnifying glass) and an inverted real image farther than F. Lenses are used in spectacles, magnifying glasses, microscopes, telescopes, cameras and in the lens of our eye. Safety: never focus sunlight with a lens onto paper, cloth or skin, and never look at the Sun through a lens.
Experiment: magnify letters on a page with a magnifying glass; move the lens away from the page and catch the inverted image of the view outside the window on white paper. Measure the focal length. Safety: do not point a magnifier toward the Sun; never leave a magnifier on paper on a summer day — fire risk.