concave
A concave lens is thicker at the edges than it is in the middle. You can see the shape of a concave lens in the Figure below . From the diagram, its clear that the lens causes rays of light to diverge, or spread apart, as they pass through it. Note that the image formed by a concave lens is on the same side of the lens as the object. It is also smaller than the object and right-side up. However, it isnt a real image. It is a virtual image. Your brain tricks you into seeing an image there. The light rays actually pass through the glass to the other side and spread out in all directions. You can explore the formation of images by a concave lens at this URL: http://phet. colorado. edu/sims/geometric-optics/geometric-optics_en. html.
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