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PHYSICS / OPTICS: ANIMATED DIAGRAM: Animation: movie of parallel light rays passing through a convex lens (converging lens) whose curvature varies through time. Focal length varies with lens curvature.

Convex lenses converge (bring together) incoming light rays to a point or focus (as compared to concave or diverging lenses which spread the light apart). In this diagrammatic animation, parallel light rays (representing a light source at infinity) from the far right (off the picture) strike the lens (viewed edge on at right of picture) made of optical glass. The lens illustrated is a biconvex lens, having both front and back surfaces bending outwards. Other combinations of surfaces are possible. The movie starts with the lens being flat (like a pane of glass) and the rays passing through it with no change in direction (i.e. the light rays remain parallel). As the lens becomes more rounded (i.e. the lens curvature or convexity increases) the rays are bent (refracted) more and the rays are converged more strongly by the lens. The point at which the rays cross is called the focus. This focal point gets closer to the lens as the lens becomes more curved. The set of focal points is called the focal plane. Lenses can also be described as a series of stacked prisms whose angles vary. 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 mammalian eye actually changes its curvature dynamically (somewhat like this animation but much more like this animation) in order to clearly focus the incoming rays (from objects at different distances from the eye) on the light sensitive retina at the back of the eye.

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