Optics:
Optics is the part of physics which involves the properties and nature of light, adding its interactions with matter and the construction of instruments that detect or use it. Optics generally describes the nature of visible, infrared light and ultraviolet. Because light is an electromagnetic wave, other types of electromagnetic radiation such as microwaves, radio waves and X-rays exhibit similar properties.
Most optical phenomena may be accounted for using the classical electromagnetic description of light. Complete electromagnetic description of light is, though, often difficult to apply in practice. Practical optics is generally done using simplified models. The most general of these, geometric optics, treats light as a collection of waves that travel in straight lines and bend when they goes through or reflect from surfaces. Physical optics is a more inclusive model of light, which includes wave effects such as interference and diffraction that may not be accounted for in geometric optics. Traditionally, the ray-based model of light was defined first, followed by the wave model of light. Progress in electromagnetic supposition in the 19th century tends to the discovery that light waves were in fact electromagnetic radiation.
Some process depends on the fact that light has both wave-like and particle-like properties. Explanation of these effects needs quantum mechanics. When taking light's particle-like properties, the light is defined as a collection of particles known as "photons". Quantum optics operates with the application of quantum mechanics to optical systems.