Thin lens

In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. If the thickness of the lens is not neglected, it is called a thick lens.

The thin lens approximation is a commonly-used approximation in geometric ray tracing that neglects the thicknesses of lenses in the optical system, which considerably simplifies the calculations needed. It is often combined with the paraxial approximation in techniques such as ray transfer matrix analysis.