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A lense is a piece of transparent material that is shaped to alter the path of light rays. There are two types of lenses, converging and diverging. Converging Lenses are also known as convex lenses, while diverging lenses are called concave. A converging lens causes parallel light rays to converge in 1 spot known as a focal point. Diverging lenses make parallel rays of light fan out. Converging lenses have a thick centre and a thin outer rim, and diverging lenses are thin in the middle and thick around the edge.

In order to understand how rays of light act when passing through a lens you must draw a ray diagram. First draw a principle axis, this axis should be vertical running top to bottom through the lense's middle. This line should be perpendicular to a horizontal line that you mark focal points on. 1 focal point should be on each side of the lens, and there should be 2 focal points in total.