Green flash

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena stemming from different causes, and some are more common than others.^[1] Green flashes may be observed from any altitude (even from an aircraft). They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well.
A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.^[2][3]

A Green flash(National Geographic Society)

Explanation

The reason for green flash optical phenomena lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper rim of the setting sun remain visible after the red rays are obstructed by the curvature of the earth.
Green flashes are enhanced by mirage, which increase the density gradient in the atmosphere and therefore, increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect to see a blue flash, but the blue is preferentially scattered out of the line of sight, and remaining light ends up looking green.
With slight magnification a green rim on the top of the solar disk may be seen on most clear-day sunsets, although the flash or ray effects require a stronger layering of the atmosphere and a mirage, which serves to magnify the green for a fraction of a second to a couple of seconds.

[edit] Types

The "green flash" description relates to a group of optical phenomena, some of which are listed below:^[1]

Type	Characteristics	Conditions	Best seen from...
Inferior-mirage flash	Joule's "last glimpse"; oval, flattened below; lasts 1 or 2 seconds	Surface warmer than the overlying air	Close to sea level
Mock-mirage flash	Indentations seem to "pinch off" a thin, pointy strip from the upper rim of the Sun; lasts 1 or 2 seconds	Atmospheric inversion layer below eye level; surface colder than air	The higher the eye, the more likely; flash is most obvious when the eye is just above the inversion.
Sub-duct flash	Large upper part of an hourglass-shaped Sun turns green for up to 15 seconds;	Observer below a strong atmospheric inversion	In a narrow height interval just below a duct (can occur at any height)
Green ray	Green beam of light either shooting up or seen immediately after sundown; usually few degrees long, lasting several seconds	Hazy air and a bright green flash acting as a light source	Sea level

The majority of flashes observed are inferior-mirage or mock-mirage effects, with the others constituting only 1% of reports. Some types not listed in the table above, such as the cloud-top flash (seen as the sun sinks into a coastal fog, or at distant cumulus clouds), are not understood.^[1]

[edit] Blue flashes

Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash".^[4] The term should not be confused with the similar usage of blue flash referring to the blue light seen in nuclear criticality accidents.

[edit] Green rim

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.^[5] The lower rim of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,^[6] but sometimes the conditions are right to see a green rim just above the horizon. The following quote describes what was probably the longest observation of a green rim, which at times could have been a green flash. It was seen on and off for 35 minutes by members of the Richard Evelyn Byrd party from the Little America exploration base in 1934:

There was a rush for the surface and as eyes turned southward, they saw a tiny but brilliant green spot where the last ray of the upper rim of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again. Altogether it remained on the horizon with short interruptions for thirty-five minutes.
When it disappeared momentarily it seemed to have been shut off by a tiny spurt, an inequality in the skyline caused by the barrier surface.
Even by moving the head up a few inches it would disappear and reappear again and after it had finally disappeared from view it could be recaptured by climbing up the first few steps of the antanea [sic] post.^[7]

For the explorers to have seen a green rim on and off for 35 minutes, there must have been some mirage effect present.
A green rim is present at every sunset, but it is too thin to be seen with the naked eye. Often a green rim changes to a green flash and back again during the same sunset. The best time to observe a green rim is about 10 minutes before sunset.^[6] That is too early to use any magnification like binoculars or a telescope to look directly at the Sun without potential harm to the eyes. (Of course, a magnified image might be projected onto a sheet of paper for safe viewing.) When the sun gets closer to the horizon, the green rim is becoming fainter because of atmospheric reddening.^[6] According to the above, it is probably correct to conclude that although a green rim is present during every sunset, a green flash is rarer because of the required mirage.
Green rim flashes

 

Persistence of vision

Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina.

The myth of persistence of vision is the mistaken belief that human perception of motion (brain centered) is the result of persistence of vision (eye centred). The myth was debunked in 1912 by Wertheimer^[1] but persists in many citations in many classic and modern film-theory texts.^[2][3][4] A more plausible theory to explain motion perception (at least on a descriptive level) are two distinct perceptual illusions: phi phenomenon and beta movement.

A visual form of memory known as iconic memory has been described as the cause of this phenomenon.^[5] Although psychologists and physiologists have rejected the relevance of this theory to film viewership, film academics and theorists generally have not. Some scientists nowadays consider the entire theory a myth.^[6]

In contrasting persistence of vision theory with phi phenomena, a critical part of understanding that emerges with these visual perception phenomena is that the eye is not a camera. In other words vision is not as simple as light registering on a medium, since the brain has to make sense of the visual data the eye provides and construct a coherent picture of reality. Joseph Anderson and Barbara Fisher argue that the phi phenomena privileges a more constructionist approach to the cinema (David Bordwell, Noël Carroll, Kirsten Thompson), whereas the persistence of vision privileges a realist approach (Andre Bazin, Christian Metz, Jean-Louis Baudry).^[6]

The discovery of persistence of vision is attributed to the Roman poet Lucretius, although he only mentions it in connection with images seen in a dream.^[7] In the modern era, some stroboscopic experiments performed by Peter Mark Roget in 1824 were also cited as the basis for the theory.^[8]

 This animated cartoon of a galloping horse is displayed at 12 drawings per second, and the fast motion is on the edge of being objectionably jerky.

Vegetative reproduction/propagation

Vegetative reproduction or vegetative propagation is a asexual or non-sexual reproduction of plants using roots. In this process a plant doesn't need the production of seeds and spores.

Types

In a wide sense, methods of vegetative propagation include cutting, Vegetative apomixis, layering, division, budding, grafting and tissue culture. Cutting is the most common artificial vegetative propagation method, where pieces of the "parent" plant are removed and placed in a suitable environment so that they can grow into a whole new plant, the "clone", which is genetically identical to the parent. Cutting exploits the ability of plants to grow adventitious roots (i.e. root material that can generate from a location other than the existing or primary root system, as in from a leaf or cut stem) under certain conditions.

Vegetative propagation is usually considered a cloning method. However, there are several cases where vegetatively propagated plants are not genetically identical. Root cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell that is genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny. Similarly, leaf cutting propagation of certain chimeral variegated plants, such as snake plant (Sansevieria trifasciata), will produce mainly nonvariegated plants.

Grafting is often not a complete cloning method because sexual seedlings are used as rootstocks. In that case only the top of the plant is clonal. In some crops, particularly apples, the rootstocks are vegetatively propagated so the entire graft can be clonal if the scion and rootstock are both clones.

Apomixis (including apospory and diplospory) is a type of reproduction that does not involve fertilisation. In flowering plants, unfertilized seeds are involved, or plantlets that grow instead of flowers. Hawkweed (Hieracium), dandelion (Taraxacum), some citrus (Citrus) and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Bulbils are sometimes formed instead of the flowers of garlic. These cases would not be vegetative reproduction because normally reproductive parts were involved. They would be considered asexual reproduction however. Vegetative reproduction involves only vegetative structures, i.e. roots, stems or leaves.

In spore-bearing plants, apospory is the asexual development of 2n gametophytes from sporophytes without undergoing meiosis or spore formation.^[1][2]

[edit]Vegetative structures

Virtually all types of shoots and roots are capable of vegetative propagation, including stems, basal shoots, tubers, rhizomes, stolons, corms, bulbs andbuds.

• The rhizome is a modified underground stem serving as an organ of vegetative reproduction, e. g. Polypody, Iris, Couch Grass and Nettles.
• Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerousgrasses, and some ferns.
• Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves.
• A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and members of the Rose Family (Rosa).
• Another type of a vegetative reproduction is the production of bulbs. Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) andtulips (Tulipa) reproduce by forming bulbs.
• Other plants like potatoes (Solanum tuberosum) and dahlia (Dahlia) reproduce by a method similar to bulbs: they produce tubers.
• Gladioli and crocuses (Crocus) reproduce by forming a bulb-like structure called a corm.
• Some orchids reproduce by the growth of keikis from the stem or cane of the parent plant.

[edit]Natural vegetative propagation

Natural vegetative reproduction is mostly a process found in herbaceous and woody perennial plants, and typically involves structural modifications of the stem, although any horizontal, underground part of a plant (whether stem or a root) can contribute to vegetative reproduction of a plant. And, in a few species (such as Kalanchoë), leaves are involved in vegetative reproduction. Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to surviveseasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony.

In a sense, this process is not one of "reproduction" but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. Of considerable interest is how this process appears to reset the aging clock.^[3]

[edit]Artificial vegetative propagation

It is very common practice to vegetatively propagate cultivars that have desirable characteristics. Man-made methods of vegetative reproduction are usually enhancements of natural processes, but range from rooting cuttings to grafting and artificial propagation by laboratory tissue culture.

In horticulture, a "cutting" is a piece that has been cut off from a mother plant and then caused to grow into a whole plant. Often this involves a piece of stem that is treated with rooting liquid or powdercontaining hormones. In some species root cuttings can produce shoot growth. When the cutting has become a self-sufficient plant, it is genetically identical to the mother plant.

A related form of regeneration is that of grafting. A stem piece or a single bud (the scion) is joined onto the stem of a plant that has roots (the rootstock), or a stem piece can be joined to a root piece. A popular use of grafting is to produce fruit trees, sometimes with more than one variety of the same fruit species growing from the same stem. Rootstocks for fruit trees are either seedlings or propagated by layering.

[edit]Cultivated plants propagated by vegetative methods

A number of commonly cultivated plants are usually propagated by vegetative means rather than by seeds. This is a listing of such plants:

African violets — leaf cuttings

Apple — grafting

Avocado — grafting

Banana — sucker removal

blackberries (Rubus occidentalis) — stem cuttings

Peach — grafting

Canna — division

Cannabis — stem cuttings

Citrus (lemon, orange, grapefruit, Tangerine) — grafting

Date — sucker removal

Fig — stem cuttings

Grapes — stem cuttings, grafting

Hops — stem cuttings

Manioc (cassava) — stem cuttings

Maple — stem cuttings, grafting

Nut crops (walnut, pecan) — grafting

Pineapple — stem cuttings

Pear — grafting

Plum — stem cuttings

Poplar — stem cuttings

Potato — stem (tuber) cuttings

Garden strawberry — runners (stolons)

Sugar cane — stem cuttings

Tea — stem cuttings

Vanilla — stem cuttings

Verbena — stem cuttings

Willow — stem cuttings