Knowing how light propagates in the physical world is crucial to understanding VR. One reason is the interface between visual displays and our eyes. Light is emitted from displays and arrives on our retinas in a way that convincingly reproduces how light arrives through normal vision in the physical world. In the current generation of VR headsets, a system of both engineered and natural lenses (parts of our eyes) guides the light. Another reason to study light propagation is the construction of virtual worlds. Section 4.1 covers basic physical properties of light, including its interaction with materials and its spectral properties. Section 4.2 provides idealized models of how lenses work. Section 4.3 then shows many ways that lens behavior deviates from the ideal model, thereby degrading VR experiences. Section 4.4 introduces the human eye as an optical system of lenses. Cameras, which can be considered as engineered eyes, are introduced in Section 4.5. Finally, Section 4.6 briefly covers visual display technologies, which emit light that is intended for consumption by human eyes.