What are the principal rays and why are they used in analyzing optical systems?
A principal ray (also known as chief ray) in an optical system is the tangential ray that starts at the edge of an object and passes through the center of the aperture stop. These rays are equivalent to rays in a pinhole camera. The distance between principal ray and the optical axis at the location of the image defines the size of the image.
The concept of principal ray is used in ray tracing method to analyze the performance of an optical system. Ray tracing allows tracing a ray of light through a system by calculating the angle of refraction/reflection at each surface.This method is useful in case of very complicated systems where Gaussian and Newtonian imaging equations cannot be used. Along with the marginal ray, the principal ray is used to define the Lagrange invariant (a measure of light propagating through an optical system).
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