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Ray (optics)
 
 
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In optics
Optics

Optics is the study of the behavior and properties of light including its optical phenomena with matter and its imaging by optical instruments....
, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing
Ray tracing (physics)

In physics, ray tracing is a method for calculating the path of waves or Subatomic particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces....
. This allows even very complex optical systems to be analyzed mathematically or simulated by computer. Ray tracing uses approximate solutions to Maxwell's equations
Maxwell's equations

In electromagnetism, James Clerk Maxwell equations are a set of four partial differential equations that describe the properties of the electric field and magnetic field fields and relate them to their sources, charge density and current density....
 that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
.






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In optics
Optics

Optics is the study of the behavior and properties of light including its optical phenomena with matter and its imaging by optical instruments....
, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing
Ray tracing (physics)

In physics, ray tracing is a method for calculating the path of waves or Subatomic particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces....
. This allows even very complex optical systems to be analyzed mathematically or simulated by computer. Ray tracing uses approximate solutions to Maxwell's equations
Maxwell's equations

In electromagnetism, James Clerk Maxwell equations are a set of four partial differential equations that describe the properties of the electric field and magnetic field fields and relate them to their sources, charge density and current density....
 that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
. Ray theory does not describe phenomena such as interference
Interference

In physics, interference is the addition of two or more waves that result in a new wave pattern.Interference usually refers to the interaction of waves which are correlated or Coherence with each other, either because they come from the same source or because they have the same or nearly the same frequency....
 and diffraction
Diffraction

Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings....
, which require wave theory (involving the phase
Phase (waves)

The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0....
 of the wave).

There are many special rays that are used in optical modelling to analyze an optical system. These are defined and described below, grouped by the type of system they are used to model.

Interaction with surfaces

  • An incident ray is a ray of light that strikes a surface. The angle between this ray and the perpendicular or normal
    Surface normal

    A surface normal, or simply normal, to a Flatness is a vector which is perpendicular to that surface. A normal to a non-flat surface at a Point P on the surface is a vector perpendicular to the Tangent space to that surface at P....
     to the surface is the angle of incidence
    Angle of incidence

    Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or Stabilizer of an airplane is installed on the fuselage, measured relative to the axis of the fuselage....
    .
  • The reflected ray corresponding to a given incident ray, is the ray that represents the light reflected by the surface. The angle between the surface normal and the reflected ray is known as the angle of reflection. The Law of Reflection says that for a specular
    Specular reflection

    Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction....
     (non-scattering) surface, the angle of reflection always equals the angle of incidence.
  • The refracted ray or transmitted ray corresponding to a given incident ray represents the light that is transmitted through the surface. The angle between this ray and the normal is known as the angle of refraction, and it is given by Snell's Law
    Snell's law

    In optics and physics, Snell's law , is a mathematical formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves, passing through a boundary between two different isotropic medium , such as water and glass....
    . Conservation of energy
    Conservation of energy

    The law of conservation of energy states that the total amount of energy in an isolated system remains constant. A consequence of this law is that energy cannot be created or destroyed....
     requires that the power in the incident ray must equal the sum of the power in the transmitted ray, the power in the reflected ray, and any power absorbed at the surface.
  • If the material is birefringent
    Birefringence

    Birefringence, or double refraction, is the decomposition of a Ray of light into two rays when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarization of the light....
    , the refracted ray may split into ordinary and extraordinary rays, which experience different indexes of refraction when passing through the birefringent material.

Optical systems

  • A meridional ray is a ray that is confined to the y-z plane, where z points along the optical axis
    Optical axis

    In optics, the term optical axis is used to define a direction along which there is some degree of rotational symmetry. It can be used in several contexts:...
     of the system, and y is perpendicular to this axis.
  • The marginal ray in an optical system is the meridional ray that starts at the point where the object crosses the optical axis, and touches the edge of the aperture stop of the system. This ray is useful, because it crosses the optical axis again at the locations where an image will be formed. The distance of the marginal ray from the optical axis at the locations of the entrance pupil
    Entrance pupil

    In an optics system, the entrance pupil is a virtual aperture that defines the area at the entrance of the system that can accept light. Rays that pass through the pupil are able to enter the optical system and pass through it to the exit ....
     and exit pupil
    Exit pupil

    In optics, the exit pupil is a virtual aperture in an optical system. Only ray which pass through this virtual aperture can exit the system. The exit pupil is the of the aperture stop in the optics that follow it....
     defines the sizes of each pupil (since the pupils are image
    Image

    An image is an artifact, usually two-dimensional , that has a similar appearance to some subject —usually a physical object or a person....
    s of the aperture stop).
  • The chief ray in an optical system is the meridional ray that starts at the edge of the object, and passes through the center of the aperture stop. This ray crosses the optical axis at the locations of the pupils. As such chief rays are equivalent to the rays in a pinhole camera. The distance between the chief ray and the optical axis at an image location defines the size of the image. The marginal and chief rays together define the Lagrange invariant
    Lagrange invariant

    In optics the Lagrange invariant is a measure of the light propagating through an optical system. It is defined by,where y and u are the marginal ray height and angle respectively, and ? and u are the chief ray height and angle....
    , which characterizes the throughput or etendue
    Etendue

    Etendue or ?tendue is a property of an optics, which characterizes how "spread out" the light is in area and angle. The ?tendue can be defined in several equivalent ways....
     of the optical system.
  • A skew ray is a ray that originates from an object point in the y-z plane, but does not propagate in this plane. Such a ray will intersect the entrance pupil at some arbitrary coordinates (xp,yp).
  • A tangential ray is a ray that intersects the entrance pupil at xp=0. This is just another name for a meridional ray.
  • A sagittal ray or transverse ray is a skew ray that intersects the pupil at yp=0.
  • A paraxial ray is a ray that makes a small angle to the optical axis of the system, and lies close to the axis throughout the system.


Fiber optics

  • A meridional ray is a ray that passes through the axis
    Optical axis

    In optics, the term optical axis is used to define a direction along which there is some degree of rotational symmetry. It can be used in several contexts:...
     of an optical fiber
    Optical fiber

    An optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers....
    .
  • A skew ray is a ray that travels in a non-planar zig-zag path and never crosses the axis
    Optical axis

    In optics, the term optical axis is used to define a direction along which there is some degree of rotational symmetry. It can be used in several contexts:...
     of an optical fiber
    Optical fiber

    An optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers....
    .
  • A guided ray
    Guided ray

    A guided ray is a Ray of light in a multimode optical fiber, which is confined by the Fiber_optics#Principle_of_operation. For Step-index profile, light entering the fiber will be guided if it falls within the acceptance cone of the fiber, that is if it makes an angle with the fiber axis that is less than the acceptance angle,...
    , bound ray, or trapped ray is a ray in a multimode optical fiber, which is confined by the core. For step index fiber
    Step-index profile

    For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the Fiber_optics#Principle_of_operation and a sharp decrease in refractive index at the core-cladding wiktionary:interface so that the cladding is of a lower refractive index....
    , light entering the fiber will be guided if it makes an angle with the fiber axis that is less than the fiber's acceptance angle
    Acceptance angle

    Acceptance angle can refer to:*Half of the angular aperture of an optical system*The angle in an optical fiber below which rays are guided rays...
    .
  • A leaky ray or tunneling ray is a ray in an optical fiber that geometric optics predicts would totally reflect
    Total internal reflection

    Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal to the surface....
     at the boundary between the core and the cladding
    Cladding

    Cladding is the covering of one material with another. It has different meanings depending on the context....
    , but which suffers loss due to the curved core boundary.

See also

  • Ray transfer matrix analysis
    Ray transfer matrix analysis

    Ray transfer matrix analysis is a type of Ray tracing technique used in the design of some optics systems, particularly lasers. It involves the construction of a ray transfer matrix which describes the optical system; tracing of a light path through the system can then be performed by multiplying this matrix with a vector space represe...
  • Paraxial approximation
    Paraxial approximation

    In geometric optics, the paraxial approximation is an approximation used in Ray tracing of light through an optical system .A paraxial ray is a Ray which makes a small angle to the optical axis of the system, and lies close to the axis throughout the system....