EYE TO THE SKY

Physics Behind the Wonders.....!

Sometimes we get caught up in the day-to-day grind that we forget how amazing and at times mysterious and terrifying the world around us really is .It is easy to take nature for granted,   its many unfathomable wonders. In this article, a selection of the most compelling and occasionally haunting optical illusions and atmospheric phenomena that the natural world has to offer are explored. When light from the Sun and the Moon enters our atmosphere, it is sometimes reflected, refracted and dispersed by tiny ice crystals, water droplets and dust to produce a whole host of fantastic atmospheric visions. Some are even more common than we realise.

Rainbow

A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

Rainbows can be full circles. However, the observer normally sees only an arc formed by illuminated droplets above the ground, and centered on a line from the sun to the observer's eye.

In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, with red on the inner side of the arc. This is caused by the light being reflected twice on the inside of the droplet before leaving it.

Aurora

An aurora , sometimes referred to as polar lights, northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in the high-latitude regions (around the Arctic and Antarctic).

Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere) due to Earth's magnetic field, where their energy is lost.

The resulting ionization and excitation of atmospheric constituents emits light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.

Aurora came from the Latin word for "dawn, morning light", since auroras were formerly thought to be the first light of dawn.

Halo

Halo is the name for a family of optical phenomena produced by light interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from coloured or white rings to arcs and spots in the sky. Many of these are near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the best known halo types are the circular halo (properly called the 22° halo), light pillars and sun dogs, but there are many more; some of them fairly common, others (extremely) rare.

The ice crystals responsible for halos are typically suspended in cirrus or cirrostratus clouds high (5–10 km, or 3–6 miles) in the upper troposphere, but in cold weather they can also float near the ground, in which case they are referred to as diamond dust. The particular shape and orientation of the crystals are responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colours because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting light between their faces, sending shafts of light in particular directions.

Sun dog

A sun dog or mock sun, formally called a parhelion is an atmospheric optical phenomenon that consists of a bright spot to the left and/or right of the Sun Two sun dogs often flank the Sun within a 22° halo.

The sun dog is a member of the family of halos, caused by the refraction of sunlight by ice crystals in the atmosphere. Sun dogs typically appear as a pair of subtly colored patches of light, around 22° to the left and right of the Sun, and at the same altitude above the horizon as the Sun. They can be seen anywhere in the world during any season, but are not always obvious or bright. Sun dogs are best seen and most conspicuous when the Sun is near the horizon.

Sun dogs are commonly caused by the refraction and scattering of light from plate-shaped hexagonal ice crystals either suspended in high and cold cirrus or cirrostratus clouds, or drifting in freezing moist air at low levels as diamond dust. The crystals act as prisms, bending the light rays passing through them with a minimum deflection of 22°. As the crystals gently float downwards with their large hexagonal faces almost horizontal, sunlight is refracted horizontally, and sun dogs are seen to the left and right of the Sun. Larger plates wobble more, and thus produce taller sundogs.

Sun dogs are red-coloured at the side nearest the Sun; farther out the colors grade through oranges to blue.

Corona

A corona is an optical phenomenon produced by the diffraction of light from the Sun or the Moon by individual small water droplets and sometimes tiny ice crystals of a cloud or on a foggy glass surface. In its full form, a corona consists of several concentric, pastel-coloured rings around the celestial object and a central bright area called aureole. The aureole is often (especially in case of the Moon) the only visible part of the corona and has the appearance of a bluish-white disk which fades to reddish-brown towards the edge. The angular size of a corona depends on the diameters of the water droplets involved: Smaller droplets produce larger coronae. For the same reason, the corona is the most pronounced when the size of the droplets is most uniform. Coronae differ from halos in that the latter are formed by refraction (rather than diffraction) from comparatively large rather than small ice crystals.

Pollen corona: - Each year trees of the Northern forests release copious clouds of pollen. The pollen grains from a given tree variety usually have very similar sizes - ideal for corona formation. 

Unlike water droplets, pollens are non spherical. Many have air sacs to assist wind dispersal and they are consequently become specially oriented as they drift in the air. The result is elongated coronae, sometimes with bright patches on their rings.

Pollens are comparatively large and so their coronae are small. Look close to a well shielded sun  or, preferably, search for them in the reflection of the sky in a pool or dark glass.

Mirage

A mirage is a naturally occurring optical phenomenon in which light rays bend to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin   mirari, meaning "to look at, to wonder at". This is the same root as for "mirror" and "to admire".  In contrast to a hallucination, a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water.

For exhausted travelers in the desert, an inferior mirage may appear to be a lake of water in the distance. An inferior mirage is called "inferior" because the mirage is located under the real object. The real object in an inferior mirage is the (blue) sky or any distant (therefore bluish) object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground in the distance.

Light rays coming from a particular distant object all travel through nearly the same air layers and all are bent over about the same amount. Therefore, rays coming from the top of the object will arrive lower than those from the bottom. The image usually is upside down, enhancing the illusion that the sky image seen in the distance is really a water or oil puddle acting as a mirror.

Diamond ring effect 

The Baily's beads effect, or diamond ring effect, is a feature of total and annular solar eclipses. As the Moon covers the Sun during a solar eclipse, the rugged topography of the lunar limb allows beads of sunlight to shine through in some places while not in others. The effect is named after Francis Baily, who explained the phenomenon in 1836. The diamond ring effect is seen when only one bead is left, appearing as a shining "diamond" set in a bright ring around the lunar silhouette.

Lunar topography has considerable relief because of the presence of mountains, craters, valleys, and other topographical features. The irregularities of the lunar limb profile (the "edge" of the Moon, as seen from a distance) are known accurately from observations of grazing occultations of stars. Astronomers thus have a fairly good idea which mountains and valleys will cause the beads to appear in advance of the eclipse. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, lasting 1–2 minutes.

Red moon

The moon does not have any light  its own ,it shines because its surface reflects sunlight. During a total lunar eclipse, the earth moves between the sun and the moon and cuts off the moon’s light supply. When this happens ,the surface of the moon takes the reddish glow instead of going completely dark.

        
The red colour of a totally eclipsed moon has prompted many people in recent years to refer to total lunar eclipses as Blood Moon.

        
The reason why the Moon takes on a reddish colour during totality is a phenomenon called Rayleigh scattering.  It is the same mechanism responsible for causing colourful sunrises and sunsets and the sky to look blue.

            
When sunlight entering the Earth's atmosphere strikes the particles that are smaller than the light's wavelength, it gets scattered into different directions. Not all colours in the light spectrum, however, are equally scattered. Colours with shorter wavelengths, especially the violet and blue colours, are scattered more strongly, so they are removed from the sunlight before it hits the surface of the Moon during a lunar eclipse. Those with longer wavelengths, like red and orange, pass through the atmosphere. This red-orange light is then bent or refracted around Earth, hitting the surface of the Moon and giving it the reddish-orange glow that total lunar eclipses are famous for.

Collected By:

                          Reshna K

                          Sahana H

                          Shilpa M P

                          Sinchana K P

Source: Internet