PHYSICS / OPTICS: Animation: movie of a convex lens whose curvature varies through time.
Convex lenses converge incoming light rays. In this animation, parallel light rays (representing a light source at infinity) from the far right (off the picture) strike the lens (at right of picture) made of optical glass. These rays are converged by the lens to a focus. An array of parallel (but highly highly translucent) screens is positioned to the left of the converging lens. The light strikes these screens and so traces the convergence of the light beam. The smaller the light spot on a screen the more the light beam has converged. As the lens becomes more rounded (i.e. the curvature increases) the rays are bent (refracted) more and so the brightest spot gets closer to the lens. This represents the shortening of the focal length as the curvature of the lens increases (i.e. the converging power of the lens increases). Convex lenses magnify images and are used in magnifying glasses, refracting telescopes, binoculars and microscopes. Lenses in cameras are similar although they often consist of a series of lenses (compound lens). The lens in the human eye actually changes its curvature dynamically in order to clearly focus the incoming rays on the light sensitive retina at the back of the eye.