Visible light is a type of electromagnetic radiation whose wavelength
lies in a relatively narrow band extending from about 400 to 700 nm.
The area of physics that is devoted to the study of light is known as optics.
This chapter is concerned with those optical phenomena that depend explicitly on the ultimate wave
nature of light, and cannot be accounted for using the well-known laws of geometric optics. (See Section 7.8.)
The branch of optics that deals with such phenomena is called wave optics.
The two most important physical phenomena that are encountered in wave optics are interference and diffraction. Interference occurs
when beams of light from multiple sources (but with similar frequencies), or multiple beams from the same source, intersect one another. Diffraction takes place, for instance, when a single beam of light passes through an opening in
an opaque screen whose spatial extent is comparable to the wavelength of the light. Actually, interference and
diffraction depend on the same underlying physics, and the distinction that is conventionally made between them is somewhat arbitrary.
