light travels in a straight line, why is this important for the technique of ray tracing?
2 answers
Drawing straight lines that only change direction at mirror and lens surfaces makes solving optics with ray-tracing a lot easier.
Rays of light travel normal to wavefronts. It only takes two rays to determine the location of images, and about ten to evaluate image quality (aberrations). Computers perform these tasks easily, giving lens designers a powerful tool. If one were to design an optics system using wavefronts, the mathematics would be much more complicated, even though the results would be the same, except for small differences due to diffraction.