Optics is the branch of physics that studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics typically describes the behavior of visible, ultraviolet, and infrared light.
Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
Most optical phenomena can be explained using the classical electromagnetic description of light. However, complete electromagnetic descriptions of light are often difficult to apply in practice. Practical optics is usually performed using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when passing through or reflecting from surfaces. Physical optics is a more comprehensive model of light, including wave effects such as diffraction and interference that cannot be explained by geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Advances in electromagnetic theory in the 19th century led to the discovery that light waves were actually electromagnetic radiation.
Some phenomena depend on the fact that light has both wave and particle properties. Explaining these effects requires quantum mechanics. When considering the particle properties of light, light is modeled as a collection of particles called "photons." Quantum optics deals with the application of quantum mechanics to optical systems.
Optical science is relevant and studied in many related disciplines, including astronomy, various fields of engineering, photography, and medicine (especially ophthalmology and optometry). Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fiber optics.