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

Concave lenses diverge (spread apart) incoming light rays. If these rays are traced backwards these imaginary lines converge to a point or vitual focus behind the lens. This is in contrast to convex (or converging lenses) which bring rays together in front of the lens to a real focus. 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 biconcave lens, having both front and back surfaces bending inwards. Other combinations of surfaces are possible. The movie starts with the lens being flat (like a thick pane of glass) and the rays pass through it with no change in direction (i.e. the light rays remain parallel). As the lens becomes more concave (i.e. the inward lens curvature or concavity increases) the rays are bent (refracted) more and the rays are diverged more strongly by the lens. The point at which the rays appear to originate is called the virtual focus and so a virtual image is created. Lenses can also be described as a series of stacked prisms whose angles vary. Concave lenses minify (make smaller) images and are used in cameras as part of a series of lenses (compound lens) in order to correct various optical deficiencies (optical aberrations).

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