paraxial ray

In geometrical optics, a paraxial ray is a ray of light that travels very close to the optical axis of a lens or mirror system. These rays are crucial for understanding ideal image formation because, unlike rays that strike the lens at greater distances from the axis, all paraxial rays originating from a single object point will converge at the same image point. This simplifies calculations, as the angles involved are small enough that trigonometric functions can be approximated by their radian values, making ray tracing more manageable. The concept of paraxial rays forms the basis for first-order imagery and is fundamental to the thin lens approximation, where the principal planes of the lens coincide.12