We start by reviewing the imaging properties of a thin lens, treating the lens as a "black box" and using ray-tracing ideas. Parallel rays of light (coming from infinity) are focussed to a point in the image focal plane of a lens. Note that this plane is on opposite sides of the lens for converging and diverging lenses. Any ray coming in parallel to the optic axis is refracted through the image focal point, and any object ray passing through the object focal point emerges from the lens parallel to the optic axis. If the lens has the same refractive index on both sides, then any ray passing through the centre of the lens is undeviated. Some examples are shown in figure 1. Note that the minimum object image separation for a converging lens is four times its focal length. Very often ray-diagrams can give a good idea of what is happening in a system.
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