The optical window is a range of wavelengths that are not blocked by the earth's

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...

. The window runs from around 300

nanometer 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re, ...

s (

ultraviolet-B Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

) up into the range the human eye can detect, roughly 400–700 nm and continues up to approximately 2

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

. Sunlight mostly reaches the ground

through Prepositions and postpositions, together called adpositions (or broadly, in traditional grammar, simply prepositions), are a class of words used to express spatial or temporal relations (''in'', ''under'', ''towards'', ''before'') or mark various ...

the optical atmospheric window; the sun is particularly active in most of this range (44% of the radiation emitted by the sun falls within the visible spectrum and 49% falls within the infrared spectrum).

Definition

The earth's atmosphere is not totally transparent and is in fact 100% opaque to many wavelengths (see plot of Earth's opacity); the wavelength ranges to which it is transparent are called atmospheric windows.

Disambiguation of the term 'optical spectrum'

Although the word ''optical'', deriving from

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

ὀπτῐκός (optikós, “of or for sight”), generally refers to something visible or visual, the term ''optical spectrum'' is used to describe the sum of the visible, the

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...

and the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

spectra (at least in this context).

The optical atmospheric window

The optical atmospheric window is the optical portion of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

that passes through the earth's atmosphere, excluding the infrared window; although, as mentioned before, the optical spectrum also includes the IR spectrum and thus the optical window should include the infrared window (8 – 14 μm), the latter is considered separate by convention, since the visible spectrum is not contained in it.

Historical importance for observational astronomy

Up until the 1940s, astronomers could only use the visible and near infrared portions of the optical window for their observations. The first great astronomical discoveries such as the ones made by the famous Italian

polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific pro ...

Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He w ...

were made using

optical telescopes An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electro ...

that received light reaching the ground through the optical window. After the 1940s, the development of radio telescopes gave rise to the even more successful field of

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comi ...

that utilized the radio window.

References

{{DEFAULTSORT:Optical Window Electromagnetic spectrum Observational astronomy

Definition

Disambiguation of the term 'optical spectrum'

The optical atmospheric window

Historical importance for observational astronomy

See also

References