Umov effect
Encyclopedia
The Umov effect, also known as Umov's law, is a relationship between the albedo
of an astronomical object, and the degree of polarization of light reflecting off it. The effect was discovered by the Russia
n physicist Nikolay Umov
in 1905, and can be observed for celestial objects such as the surface of the Moon
and the asteroid
s.
The degree of linear polarization of light P is defined by
where
and 
are the intensities of light in the directions perpendicular and parallel to the plane of a polarizer
aligned in the plane of reflection. Values of P are zero for unpolarized light, and ±1 for linearly polarized light.
Umov's law states
where α is the albedo of the object. Thus, highly reflective objects tend to reflect mostly unpolarized light, and dimly reflective objects tend to reflect polarized light. The law is only valid for large phase angle
s (angles between the incident light and the reflected light).
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of an astronomical object, and the degree of polarization of light reflecting off it. The effect was discovered by the Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
n physicist Nikolay Umov
Nikolay Umov
Nikolay Alekseevich Umov was a Russian physicist and mathematician known for discovering the concept of Umov-Poynting vector and Umov effect.-Biography:...
in 1905, and can be observed for celestial objects such as the surface of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and the asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s.
The degree of linear polarization of light P is defined by
where
and are the intensities of light in the directions perpendicular and parallel to the plane of a polarizerPolarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
aligned in the plane of reflection. Values of P are zero for unpolarized light, and ±1 for linearly polarized light.
Umov's law states
where α is the albedo of the object. Thus, highly reflective objects tend to reflect mostly unpolarized light, and dimly reflective objects tend to reflect polarized light. The law is only valid for large phase angle
Phase angle (astronomy)
Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object...
s (angles between the incident light and the reflected light).