Atmospheric optics

Through works by Roger Bacon, John Pecham, and Witelo based on Ibn al-Haytham's explanation, the Moon illusion gradually came to be accepted as a psychological phenomenon, with Ptolemy's theory being rejected in the 17th century.

[9] For over 100 years, research on the Moon illusion has been conducted by vision scientists who invariably have been psychologists specializing in human perception.

After reviewing the many different explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug concluded "No single theory has emerged victorious".

On a sunny day, Rayleigh scattering gives the sky a blue gradient, darkest around the zenith and brightest near the horizon.

[15] The sky can turn a multitude of colors such as red, orange, pink and yellow (especially near sunset or sunrise) and black at night.

Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes.

[18] Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases.

High tropospheric and non-tropospheric clouds appear mostly white if composed entirely of ice crystals and/or supercooled water droplets.

A cumulonimbus cloud emitting green is a sign that it is a severe thunderstorm,[21] capable of heavy rain, hail, strong winds and possible tornadoes.

The exact cause of green thunderstorms is still unknown, but it could be due to the combination of reddened sunlight passing through very optically thick clouds.

Yellowish clouds may occur in the late spring through early fall months during forest fire season.

Yellowish clouds caused by the presence of nitrogen dioxide are sometimes seen in urban areas with high air pollution levels.

[22] Red, orange and pink clouds occur almost entirely at sunrise and sunset and are the result of the scattering of sunlight by the atmosphere.

When the angle between the Sun and the horizon is less than 10 percent, as it is just after sunrise or just prior to sunset, sunlight becomes too red due to refraction for any colors other than those with a reddish hue to be seen.

A halo (ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by the interaction of light from the Sun or Moon with ice crystals in the atmosphere, resulting in colored or white arcs, rings or spots in the sky.

They can also form around artificial lights in very cold weather when ice crystals called diamond dust are floating in the nearby air.

The crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

[25][26] These crystals tend to become horizontally aligned as they sink through the air, causing them to refract the sunlight to the left and right, resulting in the two sun dogs.

On the giant gas planets — Jupiter, Saturn, Uranus and Neptune — other crystals form the clouds of ammonia, methane, and other substances that can produce halos with four or more sundogs.

[23] A glory is an optical phenomenon, appearing much like an iconic Saint's halo about the head of the observer, produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly sized water droplets.

A rainbow is a narrow, multicoloured semicircular arc due to dispersion of white light by a multitude of drops of water, usually in the form of rain, when they are illuminated by sunlight.

[34] A single reflection off the backs of an array of raindrops produces a rainbow with an angular size that ranges from 40° to 42° with red on the outside and blue/violet on the inside.

For colors seen by a normal human eye, the most commonly cited and remembered sequence, in English, is Isaac Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G.

[35] A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky.

In contrast to a hallucination, a mirage is a real optical phenomenon which can be captured on camera, since light rays actually are refracted to form the false image at the observer's location.

[37][38] This optical phenomenon occurs because rays of light are strongly bent when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed.

This temperature inversion is the opposite of what is normally the case; air is usually warmer close to the surface, and cooler higher up.

The Novaya Zemlya effect will give the impression that the sun is rising earlier or setting later than it actually should (astronomically speaking).

The sunlight must bend to the Earth's curvature at least 400 kilometres (250 mi) to allow an elevation rise of 5 degrees for sight of the sun disk.

The first person to record the phenomenon was Gerrit de Veer, a member of Willem Barentsz' ill-fated third expedition into the polar region.