The Umov effect, also known as Umov's law, is a relationship between the
albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, 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 ...
of an astronomical object, and the
degree of polarization of light reflecting off it.
The effect was discovered by the
Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
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.
Bi ...
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 of ...
and the
asteroid
An asteroid is a minor planet of the 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 or icy bodies with no atmosphere.
...
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
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
:
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