The Umov effect, also known as Umov's law, is a relationship between the

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...

of an astronomical object, and the degree of

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

of light reflecting off it. The effect was discovered by the

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eigh ...

n physicist

Nikolay Umov Nikolay Alekseevich Umov (russian: Никола́й Алексе́евич У́мов; January 23, 1846 – January 15, 1915) was a Russian physicist and mathematician known for discovering the concept of Umov-Poynting vector and Umov effect. ...

in 1905, and can be observed for celestial objects such as the surface of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...

and the

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...

s. The ''degree of linear polarization'' of light ''P'' is defined by :

P = \frac\ ,

where

I_\perp

and

I_\,

are the intensities of light in the directions perpendicular and parallel to the plane of a

polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well ...

aligned in the plane of reflection. Values of ''P'' are zero for unpolarized light, and ±1 for linearly polarized light. Umov's law states :

P \propto \frac\ ,

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 angles (angles between the incident light and the reflected light).

References

{{reflist Observational astronomy Planetary science