Optical path (OP) is the trajectory that a light ray follows as it propagates through an

optical medium An optical medium is material through which light and other electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. Properties The ...

. The geometrical optical-path length or simply geometrical path length (GPD) is the

length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Inte ...

of a segment in a given OP, i.e., the

Euclidean distance In mathematics, the Euclidean distance between two points in Euclidean space is the length of a line segment between the two points. It can be calculated from the Cartesian coordinates of the points using the Pythagorean theorem, therefore o ...

integrated along a ray between any two points. The mechanical length of an optical device can be reduced to less than the GPD by using

folded optics Folded optics is an optical system in which the beam is bent in a way to make the optical path much longer than the size of the system. This allows the resulting focal length of the objective to be greater than the physical length of the optical d ...

. The ''

optical path length In optics, optical path length (OPL, denoted ''Λ'' in equations), also known as optical length or optical distance, is the product of the geometric length of the optical path followed by light and the refractive index of homogeneous medium throu ...

'' in a homogeneous medium is the GPD multiplied by the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

of the medium.

Factors affecting optical path

Path of light in medium, or between two media is affected by the following: * Reflection **

Total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflecte ...

Refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...

* Dispersion of light * Absorption

Simple materials used

Lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

* Prisms *

Mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

s * Transparent materials (e.g.

optical filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...

s) * Translucent materials (e.g. frosted glass) * Opaque materials

References

Physical optics Geometrical optics {{Optics-stub