Light Travels in Straight Lines

In the geometric-optics approximation, light rays travel in straight lines through a uniform material such as air, water or glass. Diffraction is neglected; this works well when the relevant sizes are much larger than a wavelength. We model a thin beam as a ray: an arrow showing the direction energy flows.

An opaque object blocks rays and casts a shadow. A small source can give a sharp geometric edge; extended sources produce partial shadows, and diffraction softens edges on small scales. Everything in this course is built from tracing rays: fire one from a source and follow where it goes.