Reflection

Summary

Optics is the study of the nature and behavior of light.
It can be divided into subdisciplines based on the type of model used to describe light.
- In physical optics, light is assumed to behave like a classical wave.
- In quantum optics, light is assumed to have both wave and particle properties.
  - Particles of light are called photons.
- In geometric optics, light is assumed to travel in a definite direction with relatively little diffraction.
  - This behavior is known as rectilinear propagation.
  - The path of propagation of a light wave is a geometric ray.
    - The rays of geometric optics …
      - are perpendicular to the wave fronts of physical optics.
      - indicate the most probable path of the photons of quantum optics.
    - A ray is the path of least action connecting two points in space and is also …
      - the path of least time (the quickest path)
      - the path of least distance (the shortest path)
      - unique and therefore reversible
        
        The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.
    - Rays are …
      - straight lines in empty euclidean space
      - geodesics in general relativistic space-time
    - The eye can see something only if a ray from the object reaches the eye.
Interface
- An interface is the boundary between …
  - two different media.
  - two regions of a medium with different characteristics such as …
    - density (which is often related to temperature)
    - concentration of solute (salinity, for example)
    - mechanical stress
- When an incident ray meets an interface it will be partially
  - reflected
    - Reflected rays obey the law of reflection described in this section of this book.
  - transmitted
    - Transmitted rays obey Snell's law, which is described in the next section of this book.
  - absorbed
    - Absorbed rays obey the law of conservation of energy. (The energy of the ray is not destroyed, but changes form.)
- Angles in geometric optics are measured with respect to a line normal to the interface.
  - The angle of incidence is the angle between the incident ray and the normal.
  - The angle of reflection is the angle between the reflected ray and the normal.
  - The angle of refraction is the angle between the transmitted ray and the normal.
Reflection
- Law of reflection: The angle of incidence equals the angle of reflection.
  - The law of reflection can be derived from the principle of least action.

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