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Basic Optical Principles
Lenses as Prisms

When two prisms are placed base to base they can be said to form a plus lens. Plus lenses cause parallel rays of light to converge or come together. The images formed by a plus lens are called a real images because the rays of light actually do come together to form a real focal point. The distance from the lens to the focal point is called the focal length. If the focal length of a given lens is equal to 1 meter, that lens is said to have a power of 1 diopter. The power of the lens above is equal to + 1.00 diopter since it is a converging or plus lens.

When two prisms are placed apex to apex they can be said to form a minus lens. Minus lenses cause parallel rays of light to diverge which means they never actually come together as they do in a plus lens. However, if we were to extend the rays backward as in the above diagram, they would eventually meet at a point which is called a virtual focal point since it doesn�t actually exist. The image it forms is called a virtual image. The distance from the lens to the virtual focal point is called the focal length. If this distance is equal to one meter, the lens is said to have a power of �1.00 diopter.


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