# Optics

Optics is the branch of physics that deals with the properties and behavior of light, as well as its interaction with matter. Because of the wave nature of light and the fact that light can manifest itself as forms of electromagnetic radiation, optics is often studied as a sub-field of electromagnetism. Under this framework, nearly all the behaviors of light can be explained by Maxwell's Equations.

Optics can be informally divided in classical optics and modern optics. Classical optics is further divided into geometrical optics and physical optics.

## Classical optics

In this scheme, light is modeled as an electromagnetic wave. The color of the light is determined by the frequency of the wave (Number of oscillations per second), and the intensity is determined by the amplitude of the wave.

### Geometrical optics

Light is described in terms of rays. The ray is an abstraction that tells us the direction in which the light propagates. According to the Fermat principle, the path taken between two points by a light ray is the path that can be traversed in the least time. Geometrical optics can explain the phenomena of reflection and refraction (How the direction of light changes when it passes to medium with a different speed of light). Since geometric optics only accounts for the direction of light, and ignore its wave like properties, it cannot explain the properties of light that are consequence of this wave behavior, like interference, diffraction and polarization.

### Physical optics

Light is fully described as a wave. It is based on the Huygens principle, which states that every point of an advanced wave front is the center of a new disturbance. Some of the properties of light that can be described with physical optics are:

• Interference: The addition of two or more waves to form a new wave pattern.
• Diffraction: The bending of waves around small obstacles and the spreading out of waves when they pass small openings.
• Polarization: Describes the orientation of the oscillations of the electromagnetic waves.

## Modern optics

Incorporates the theory of quantum mechanics into optics. Light is described as a beam of particles called photons. Because of the wave matter duality of quantum theory, this photons have wave like properties. In this scheme, the frequency of the photons determines the color of the light, and the intensity of the light is determined by the number of photons. Modern optics has a large number of practical applications, like the laser and optic fibers.