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) guide the light. Another reason to study light propagation is the construction of virtual worlds. Chapter 3 covered purely geometric aspects of modeling. The next logical step is to model the physics of light propagation through virtual worlds; this will be continued in Chapter 7, which describes what should be rendered on the visual display. Finally, light propagation is also helpful to understanding how cameras work, which provides another way present a virtual world: Through panoramic videos. Cameras are also important for tracking, which will be discussed in Section 9.3.
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, before eyes and human vision are covered in much more detail in Chapter 5. Cameras, which can be considered as engineered eyes, are introduced in Section 4.5.