Saturday, July 17, 2010

Persian Cats

Persian (cat)



The Persian (Persian: گربه پارسی Persian cat) is a longhaired cat characterized by its round face and shortened muzzle. One of the oldest cat breeds, it takes its name from its place of origin: Persia (Iran). Recognized by the cat fancy since the late 19th century, it was developed first by the English, and then mainly by American breeders after the Second World War. In Britain, it is called the Longhair or Persian Longhair.

The selective breeding carried out by breeders has allowed the development of a wide variety of coat colors, but has also led to the creation of increasingly flat-faced Persians. Favored by fanciers, this head structure can bring with it a number of health problems. Like the case with the Siamese breed, there have been efforts by some breeders to preserve the older type of cat with a more pronounced muzzle, which is more popular with the general public. The hereditary polycystic kidney disease is prevalent in the breed, affecting almost half the population in some countries.

The placid and homely nature of the Persian confers a propensity for apartment living. It has been the most popular breed in the United States for many years but its popularity has seen a decline in Britain and France.

Origin

In general, it's not clear when longhaired cats first appeared, as there are no African Wildcats, which are believed to be ancestors of domesticated cats, with long fur. There were claims in the 1800s that the gene responsible for long hair was introduced through hybridization with the Pallas cat, however, research in the early 1900s refutes this theory.





An Angora/Persian from "The Royal Natural History" (1894)

The first documented ancestors of the Persian were imported from Khorasan, Persia into Italy in 1620 by Pietro della Valle, and from Angora, Turkey into France by Nicholas-Claude Fabri de Peiresc at around the same time. The Khorasan cats were grey coated while those from Angora were white. From France, they soon reached Britain.[1] Longhaired cats were also imported to Europe from Afghanistan, Burma, China and Russia. Interbreeding of the various types were common especially between Angoras and Persians.[2]

Recent genetic research indicates that present day Persians are related not to cats from the Near East but to cats from Western Europe. The researchers stated that "Even though the early Persian cat may have in fact originated from ancient Persia, the modern Persian cat has lost its phylogeographical signature."[3]

Development

Persians and Angoras







Top: Blue Persian. Prize-winner at Westminster in 1899.

Bottom: Silver Persian. Winner of multiple leading cat shows.

The Persian was presented at the first cat show in Crystal Palace, London in 1871. As specimens closer to the Persian conformation became the more popular types, attempts were made to differentiate it from the Angora.[2] The first breed standards (then known as points of excellence) was issued in 1889 by Harrison Weir, the creater of the first cat show. He stated that the Persian differed from the Angora in the tail being longer, hair more full and coarse at the end and head larger, with less pointed ears.[4] Not all cat fanciers agree with the distinction of the two types and in the 1903 book "The Book of the Cat" Francis Simpson states that "the distinctions, apparently with hardly any difference, between Angoras and Persians are of so fine a nature that I must be pardoned if I ignore the class of cat commonly called Angora".[5]

Dorothy Bevill Champion lays out the difference between the two types in the 1909 Everybody's Cat Book:[6]

Our pedigree imported long-hairs of to-day are undoubtedly a cross of the Angora and Persian ; the latter possesses a rounder head than the former, also the coat is of quite a different quality. The coat of the Persian consists of a woolly under coat and a long, hairy outer coat. In summer it loses all the thick underwool, and only the long hair remains. The hair is also somewhat shorter on the shoulders and upper part of the hind legs.

Now, the Angora has a very different coat, consisting of long, soft hair, hanging in locks, inclining to a slight curl or wave on the under parts of the body. The hair is also much longer on the shoulders and hind legs than the Persian, this being a great improvement; but the Angora fails to the Persian in head, the former having a more wedge-shaped head, whereas that of the modern Persian excels in roundness.

Of course. Angoras and Persians have been constantly crossed, with a decided improvement to each breed; but the long-haired cat of to-day is decidedly more Persian-bred than Angora.

Champion lamented the lack of distinction among various long-haired types by English fanciers, who in 1887, decided to group them under the umbrella term "Long-haired Cats".[2]

Peke-face and ultra-typing

In the late 1950s a spontaneous mutation in red and red tabby Persians gave rise to the peke-faced Persian, named after the flat-faced Pekingese dog. It was registered as a breed by the CFA but fell out of favor by the mid 1990s due to serious health issues. In fact, only 98 were registered between 1958 and 1995. Despite this, breeders took a liking to the look and started breeding towards the peke-face look. The over-accentuation of the breed's characteristics by selective breeding (called extreme- or ultra-typing) produced results similar to the peke-faced Persians. The term peke-face has been used to refer to the ultra-typed Persian but it is properly used only to refer to red and red tabby Persians bearing the mutation. Many fanciers and CFA judges considered the shift in look "a contribution to the breed"[1][7][8][9]



A Persian with a visible muzzle in contrast with a Persian with its forehead, nose and chin in vertical alignment, as called for by CFA's 2007 breed standard. The shorter the muzzle, the higher the nose tends to be. UK standards penalizes Persians whose nose leather extends above the bottom edge of the eye.

In 1958, breeder and author P. M. Soderberg wrote in "Pedigree Cats, Their Varieties, breeding and Exhibition"[9]

Perhaps in recent times there has been a tendency to over-accentuate this type of short face, with the result that a few of the cats seen at shows have faces which present a peke-like appearance. This is a type of face which is definitely recognized in the United States, and helps to form a special group within the show classification for the [Persian] breed. There are certainly disadvantages when the face has become too short, for this exaggeration of type is inclined to produce a deformity of the tear ducts, and running eyes may be the result. A cat with running eyes will never look at its best because in time the fur on each side of the nose becomes stained, and thus detracts from the general appearance [...] The nose should be short, but perhaps a plea may be made here that the nose is better if it is not too short and at the same time uptilted. A nose of this type creates an impression of grotesqueness which is not really attractive, and there is always a danger of running eyes





A smoke Persian with moderate features.

While the looks of the Persian changed, the Persian Breed Council's standard for the Persian had remained basically the same. The Persian Breed Standard is, by its nature, somewhat open-ended and focused on a rounded head, large, wide-spaced round eyes with the top of the nose leather placed no lower than the bottom of the eyes. The standard calls for a short, cobby body with short, well-boned legs, a broad chest, and a round appearance, everything about the ideal Persian cat being "round". It was not until the late 1980s that standards were changed to limit the development of the extreme appearance.[citation needed] In 2004, the statement that muzzles should not be overly pronounced was added to the breed standard.[10] The standards were altered yet again in 2007, this time to reflect the flat face, and it now states that the forehead, nose, and chin should be in vertical alignment.[11]

In the UK, the standard was changed by the Governing Council of the Cat Fancy in the 1990s to disqualify Persians with the "upper edge of the nose leather above the lower edge of the eye" from Certificates or First Prizes in Kitten Open Classes.[12][13]

While ultra-typed cats do better in the show ring, the public seems to prefer to less extreme older "doll face" types.[1]

Himalayan





The Himalayan or Colorpoint Longhair was created by crossing the Persian with the Siamese. This crossing also introduced the chocolate and lilac color into solid colored Persians.

In 1950, the Siamese was crossed with the Persian to create a breed with the body type of the Persian but colorpoint pattern of the Siamese. It was named Himalayan, after other colorpoint animals such as the Himalayan rabbit. In the UK the breed was recognized as the Colorpoint Longhair. The Himalayan stood as a separate breed in the US until 1984, when the CFA merged it with the Persian, to the objection of the breed councils of both breeds. Some Persian breeders were unhappy with the introduction of this "hybrid" into their "pure" Persian lines.[14][15]

The CFA set up the registration for Himalayans in a way that breeders would be able to discern a Persian with Himalayan ancestry just by looking at the pedigree registration number. This was to make it easy for breeders who do not want Himalayan blood in their breeding lines to avoid individuals who, while not necessarily exhibiting the colorpoint pattern, may be carrying the point coloration gene recessively. Persians with Himalayan ancestry has registration numbers starting with 3 and are commonly referred to by breeders as colorpoint carriers (CPC) or 3000-series cats, although not all will actually carry the recessive gene. The Siamese is also the source for the chocolate and lilac color in solid Persians.[16][17]

Exotic Shorthair





The Exotic Shorthair is similar to the Persian in temperament and type, with the exception of its short, dense coat.

The Persian was used as an outcross secretly by some American Shorthair (ASH) breeders in the late 1950s to "improve" their breed. The hybrid look gained recognition in the show ring but other breeders unhappy with the changes successfully pushed for new breed standards that would disqualify ASH that showed signs of hybridization.

One ASH breeder who saw the potential of the Persian/ASH cross proposed and eventually managed to get the CFA to recognize them as a new breed in 1966, under the name Exotic Shorthair. Regular outcrossing to the Persian has made present day Exotic Shorthair similar to the Persian in every way, including temperament and conformation, with the exception of the short dense coat. It has even inherited much of the Persian's health problems. The easier to manage coat has made some label the Exotic Shorthair the lazy person’s Persian.

Because of the regular use of Persians as outcrosses, some Exotics may carry a copy of the recessive longhair gene. When two such cats mate, there is a one in four chance of each offspring being longhaired. Ironically, longhaired Exotics are not considered Persians by CFA, although The International Cat Association accepts them as Persians. Other associations register them as a separate Exotic Longhair breed.[18]

Toy and teacup Persians

[9] A number of breeders produce small-statured Persians under a variety of names. The generic terms is "toy" and "teacup" Persians (terms borrowed from the dog fancy), but the individual lines are called "palm-sized", "pocket", "mini" and "pixie". Currently none are recognised as breeds by major registries and each breeder sets their own standards for size. [19]

Chinchilla



A doll face silver Persian

In the USA, there was an attempt to establish the Silver Persian as a separate breed called the Sterling, but it was not accepted. Silver and Golden longhaired cats, recognized by CFA more specially as Chinchilla Silvers, Shaded Silvers, Chinchilla Goldens, or Shaded Goldens, are judged in the Persian category of cat shows. In South Africa, the attempt to separate the breed was more successful; the Southern African Cat Council (SACC) registers cats with five generations of purebred Chinchilla as a Chinchilla Longhair. The Chinchilla Longhair has a slightly longer nose than the Persian, resulting in healthy breathing and less eye tearing. Its hair is translucent with only the tips carrying black pigment, a feature that gets lost when out-crossed to other colored Persians. Out-crossing also may result in losing nose and lip liner, which is a fault in the Chinchilla Longhair breed standard. One of the distinctions of this breed is the blue-green or green eye color only with kittens having blue or blue-purple eye color.[citation needed]

Doll Face

The Doll Face Persian is also called the traditional persian. The Persian Standard did not change in about the last twenty years, only some breeders and judges in America started to interpret the standard differently. The short nose and clear break became shorter and higher. This resulted eventually in the 'peke face' Persian, now forbidden because of the breed's health problems.

Popularity



The popularity of the Persian (blue line) in the UK has declined for the past two decades.

The Persian is the most popular breed of pedigree cats in the United States.[20] In the UK, registration numbers have dwindled since the early 1990s and the Persian lost its top spot to the British Shorthair in 2001. As of 2008, it was the 5th most popular breed, behind the British Shorthair, Siamese and Bengal.[21] In France, the Persian is the only breed whose registration declined between 2003 and 2007, dropping by more than a quarter.[22]

The most popular varieties according to CFA registration data are Seal Point, Blue Point, Flame Point and Tortie Point Himalayan, followed by Black-White, Shaded Silvers and Calico Persians.[20]

Characteristics


A Grand Champion chocolate Persian.

A show-quality Persian has an extremely long and thick coat, short legs, a wide head with the ears set far apart, large eyes, and an extremely shortened muzzle. The breed was originally established with a short muzzle, but over time, this characteristic has become extremely exaggerated, particularly in North America. Persian cats can have any color or markings including pointed, golden, tortoiseshell, blue, and tabby.

The Persian is generally described as a quiet cat. Homely and placid, it adapts well to apartment life. Himalayans tend to be more active due to the influence of the Siamese. One study compared cat owners' perception of their cats and Persians rated higher than non-pedigree cats on closeness and affection to owners, friendliness towards strangers, cleanliness, predictability, vocalization and fussiness over food.[23]

Health

Longevity of the Persian is usually between 10 and 15 years on average.[citation needed] The modern brachycephalic Persian has a large rounded skull and shortened face and nose. This facial conformation makes the breed prone to breathing difficulties, skin and eye problems and birthing difficulties. Anatomical abnormalities associated with brachycephalic breeds can cause shortness of breath.[24] Malformed tear ducts causes epiphora, an overflow of tears onto the face, which is common but primarily cosmetic. It can be caused by other more serious conditions though. Entropion, the inward folding of the eyelids, causes the eyelashes to rub against the cornea, and can lead to tearing, pain, infection and cornea damage. Similarly, in upper eyelid trichiasis or nasal fold trichiasis, eyelashes/hair from the eyelid and hair from the nose fold near the eye grow in a way which rubs against the cornea.[25] Dystocia, an abnormal or difficult labor, is relatively common in Persians.[26] Consequently, stillbirth rate is higher than normal, ranging from 16.1% to 22.1%, and one 1973 study puts kitten mortality rate (including stillborns) at 29.2%.[27]. A veterinary study in 2010 documented the serious health problems caused by the brachycephalic head.[28]

As a consequence of the BBC program Pedigree Dogs Exposed, cat breeders have too come under pressure from veterinary and animal welfare associations, with the Persian singled out as one of the breeds most affected by health problems.[29]. Animal welfare proponents have suggested changes to breed standards to prevent diseases caused by over or ultra-typing, and prohibiting the breeding of animals outside the set limits.[30] Apart from the GCCF standard that limits high noses, TICA and FIFe standards require nostrils to be open, with FIFe stating that nostrils should allow "free and easy passage of air." Germany's Animal Welfare Act too prohibits the breeding of brachycephalic cats in which the tip of the nose is higher than the lower eyelids.[28]

Polycystic kidney disease (PKD) which causes kidney failure in affected adult cats has an incidence rate of 36 - 49% in the Persian breed.[31] Cysts develop and grow in the kidney over time, replacing kidney tissues and enlarging the kidney. Kidney failure develops later in life, at an average age of 7 years old (ranging from 3 to 10 years old). Symptoms include excessive drinking and urination, reduced appetite, weight loss and depression.[32] The disease is autosomal dominant and ultrasound or DNA screening to remove affected individuals from the breeding pool has allowed some lines and catteries to drastically reduce or eliminate the incidence of the disease.[33]

Hypertrophic cardiomyopathy (HCM) is a common heart disease in all cats. It is hereditary in the Maine Coon and American Shorthair, and likely the Persian. The disease causes thickening of the left heart chamber, which can in some instances lead to sudden death. It tends to affect males and mid to old-aged individuals. Reported incidence rate in Persians is 6.5%.[34] Unlike PKD which can be detected even in very young cats, heart tests for HCM have to be done regularly in order to effective track and/or remove affected individuals and their offspring from the breeding pool.[35]

Early onset Progressive retinal atrophy is a degenerative eye disease with an autosomal recessive mode of inheritance in the Persian.[36] Despite a belief among some breeders that the disease is limited to Chocolate and Himalayan lines, there is no apparent link between coat color in Persians and the development of PRA.[37] Basal cell carcinoma is a skin cancer which shows most commonly as a growth on the head, back or upper chest. While often benign, rare cases of malignancy tends to occur in Persians.[38] Blue smoke Persians are predisposed to Chédiak-Higashi syndrome. White cats, including white Persians, are prone to deafness, especially those with blue eyes.[39] Persians are more prone to side effects of ringworm drug Griseofulvin.[40]

As with in dogs, hip dysplasia affects larger breeds such as Maine Coons and Persians. But the small size of cats means that they tend not to be as affected by the condition.[38] Persians are susceptible to malocclusion (incorrect bite), which can affect their ability to grasp, hold and chew food.[38] Even without the condition the flat face of the Persian can make picking up food difficult, so much so that specially shaped kibble have been created by pet food companies to cater to the Persian.[41]

Other conditions which the Persian is predisposed to are listed below:[42]

· Dermatological - Primary seborrhoea, Idiopathic periocular crusting, Dermatophytosis, Facial fold pyoderma, Idiopathic facial dermatitis (aka dirty face syndrome), Multiple epitrichial cysts(eyelids)

· Ocular - Coloboma, Lacrimal punctal aplasia, Corneal sequestrum, Congenital cataract

· Urinary - Calcium oxalate urolithiasis (Feline lower urinary tract disease)

· Reproductive - Cryptorchidism

· Gastrointestinal - Congenital portosystemic shunt, Congenital polycystic liver disease (associated with PKD)

· Cardiovascular - Peritoneopericardial diaphragmatic hernia

· Immunological - Systemic lupus erythematosus

· Neurological - Alpha-mannosidosis

· Neoplastic - Basal cell carcinoma, Sebaceous gland tumours



Grooming


In a "lion cut", the cat's body is shaved, leaving fur on the head, legs and tip of the tail intact. It may be done to remove matted fur, reduce the need for grooming, keep the cat cool in warm weather or for aesthetic reasons.

Since Persian cats have long, thick dense fur that they cannot effectively keep clean, they need regular grooming to prevent matting. To keep their fur in its best condition, they must be bathed regularly, dried carefully afterwards, and brushed thoroughly every day. An alternative is to shave the coat. Their eyes may require regular cleaning to prevent crust buildup and tear staining.

Rain

What began as a buck season with lots of promise has degenerated into a long, wet slog.


It has rained nearly every day of this year’s West Virginia firearm season for buck deer, and that much rain never augurs well for hunters’ success. At this point in the season, I wouldn’t be surprised to see the harvest fall short of last year’s total of 68,385. That would be a shame, as DNR officials were expecting a kill in the 70,000-to-75,000 range.


Rainbow

What is a rainbow?



Author Donald Ahrens in his text Meteorology Today describes a rainbow as "one of the most spectacular light shows observed on earth". Indeed the traditional rainbow is sunlight spread out into its spectrum of colors and diverted to the eye of the observer by water droplets. The "bow" part of the word describes the fact that the rainbow is a group of nearly circular arcs of color all having a common center.

Where is the sun when you see a rainbow?

This is a good question to start thinking about the physical process that gives rise to a rainbow. Most people have never noticed that the sun is always behind you when you face a rainbow, and that the center of the circular arc of the rainbow is in the direction opposite to that of the sun. The rain, of course, is in the direction of the rainbow.

What makes the bow?

A question like this calls for a proper physical answer. We will discuss the formation of a rainbow by raindrops. It is a problem in optics that was first clearly discussed by Rene Descartes in 1637. An interesting historical account of this is to be found in Carl Boyer's book, The Rainbow From Myth to Mathematics. Descartes simplified the study of the rainbow by reducing it to a study of one water droplet and how it interacts with light falling upon it.

He writes:"Considering that this bow appears not only in the sky, but also in the air near us, whenever there are drops of water illuminated by the sun, as we can see in certain fountains, I readily decided that it arose only from the way in which the rays of light act on these drops and pass from them to our eyes. Further, knowing that the drops are round, as has been formerly proved, and seeing that whether they are larger or smaller, the appearance of the bow is not changed in any way, I had the idea of making a very large one, so that I could examine it better.



Descarte describes how he held up a large sphere in the sunlight and looked at the sunlight reflected in it. He wrote "I found that if the sunlight came, for example, from the part of the sky which is marked AFZ



and my eye was at the point E, when I put the globe in position BCD, its part D appeared all red, and much more brilliant than the rest of it; and that whether I approached it or receded from it, or put it on my right or my left, or even turned it round about my head, provided that the line DE always made an angle of about forty-two degrees with the line EM, which we are to think of as drawn from the center of the sun to the eye, the part D appeared always similarly red; but that as soon as I made this angle DEM even a little larger, the red color disappeared; and if I made the angle a little smaller, the color did not disappear all at once, but divided itself first as if into two parts, less brilliant, and in which I could see yellow, blue, and other colors ... When I examined more particularly, in the globe BCD, what it was which made the part D appear red, I found that it was the rays of the sun which, coming from A to B, bend on entering the water at the point B, and to pass to C, where they are reflected to D, and bending there again as they pass out of the water, proceed to the point ".



This quotation illustrates how the shape of the rainbow is explained. To simplify the analysis, consider the path of a ray of monochromatic light through a single spherical raindrop. Imagine how light is refracted as it enters the raindrop, then how it is reflected by the internal, curved, mirror-like surface of the raindrop, and finally how it is refracted as it emerges from the drop. If we then apply the results for a single raindrop to a whole collection of raindrops in the sky, we can visualize the shape of the bow.



The traditional diagram to illustrate this is shown here as adapted from Humphreys, Physics of the Air. It represents the path of one light ray incident on a water droplet from the direction SA. As the light beam enters the surface of the drop at A, it is bent (refracted) a little and strikes the inside wall of the drop at B, where it is reflected back to C. As it emerges from the drop it is refracted (bent) again into the direction CE. The angle D represents a measure of the deviation of the emergent ray from its original direction. Descartes calculated this deviation for a ray of red light to be about 180 - 42 or 138 degrees.



The ray drawn here is significant because it represents the ray that has the smallest angle of deviation of all the rays incident upon the raindrop. It is called the Descarte or rainbow ray and much of the sunlight as it is refracted and reflected through the raindrop is focused along this ray. Thus the reflected light is diffuse and weaker except near the direction of this rainbow ray. It is this concentration of rays near the minimum deviation that gives rise to the arc of rainbow.



The sun is so far away that we can, to a good approximation, assume that sunlight can be represented by a set of parallel rays all falling on the water globule and being refracted, reflected internally, and refracted again on emergence from the droplet in a manner like the figure. Descartes writes



I took my pen and made an accurate calculation of the paths of the rays which fall on the different points of a globe of water to determine at which angles, after two refractions and one or two reflections they will come to the eye, and I then found that after one reflection and two refractions there are many more rays which can be seen at an angle of from forty-one to forty-two degrees than at any smaller angle; and that there are none which can be seen at a larger angle" (the angle he is referring to is 180 - D).



A typical raindrop is spherical and therefore its effect on sunlight is symmetrical about an axis through the center of the drop and the source of light (in this case the sun). Because of this symmetry, the two-dimensional illustration of the figure serves us well and the complete picture can be visualized by rotating the two dimensional illustration about the axis of symmetry. The symmetry of the focusing effect of each drop is such that whenever we view a raindrop along the line of sight defined by the rainbow ray, we will see a bright spot of reflected/refracted sunlight. Referring to the figure, we see that the rainbow ray for red light makes an angle of 42 degrees between the direction of the incident sunlight and the line of sight. Therefore, as long as the raindrop is viewed along a line of sight that makes this angle with the direction of incident light, we will see a brightening. The rainbow is thus a circle of angular radius 42 degrees, centered on the antisolar point, as shown schematically here.



We don't see a full circle because the earth gets in the way. The lower the sun is to the horizon, the more of the circle we see -right at sunset, we would see a full semicircle of the rainbow with the top of the arch 42 degrees above the horizon. The higher the sun is in the sky, the smaller is the arch of the rainbow above the horizon.



What makes the colors in the rainbow?

The traditional description of the rainbow is that it is made up of seven colors - red, orange, yellow, green, blue, indigo, and violet. Actually, the rainbow is a whole continuum of colors from red to violet and even beyond the colors that the eye can see.

The colors of the rainbow arise from two basic facts:



Sunlight is made up of the whole range of colors that the eye can detect. The range of sunlight colors, when combined, looks white to the eye. This property of sunlight was first demonstrated by Sir Isaac Newton in 1666.

Light of different colors is refracted by different amounts when it passes from one medium (air, for example) into another (water or glass, for example).

Descartes and Willebrord Snell had determined how a ray of light is bent, or refracted, as it traverses regions of different densities, such as air and water. When the light paths through a raindrop are traced for red and blue light, one finds that the angle of deviation is different for the two colors because blue light is bent or refracted more than is the red light. This implies that when we see a rainbow and its band of colors we are looking at light refracted and reflected from different raindrops, some viewed at an angle of 42 degrees; some, at an angle of 40 degrees, and some in between. This is illustrated in this drawing, adapted from Johnson's Physical Meteorology. This rainbow of two colors would have a width of almost 2 degrees (about four times larger than the angular size as the full moon). Note that even though blue light is refracted more than red light in a single drop, we see the blue light on the inner part of the arc because we are looking along a different line of sight that has a smaller angle (40 degrees) for the blue.

Ana excellent laboratory exercise on the mathematics of rainbows is here, and F. K. Hwang has produced a fine Java Applet illustrating this refraction, and Nigel Greenwood has written a program that operates in MS Excel that illustrates the way the angles change as a function of the sun's angle.



What makes a double rainbow?

Sometimes we see two rainbows at once, what causes this? We have followed the path of a ray of sunlight as it enters and is reflected inside the raindrop. But not all of the energy of the ray escapes the raindrop after it is reflected once. A part of the ray is reflected again and travels along inside the drop to emerge from the drop. The rainbow we normally see is called the primary rainbow and is produced by one internal reflection; the secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50 degrees° rather than the 42°degrees for the red primary bow. Blue light emerges at an even larger angle of 53 degrees°. his effect produces a secondary rainbow that has its colors reversed compared to the primary, as illustrated in the drawing, adapted from the Science Universe Series Sight, Light, and Color.

It is possible for light to be reflected more than twice within a raindrop, and one can calculate where the higher order rainbows might be seen; but these are never seen in normal circumstances.



Why is the sky brighter inside a rainbow?

Notice the contrast between the sky inside the arc and outside it. When one studies the refraction of sunlight on a raindrop one finds that there are many rays emerging at angles smaller than the rainbow ray, but essentially no light from single internal reflections at angles greater than this ray. Thus there is a lot of light within the bow, and very little beyond it. Because this light is a mix of all the rainbow colors, it is white. In the case of the secondary rainbow, the rainbow ray is the smallest angle and there are many rays emerging at angles greater than this one. Therefore the two bows combine to define a dark region between them - called Alexander's Dark Band, in honor of Alexander of Aphrodisias who discussed it some 1800 years ago!

What are Supernumerary Arcs?

In some rainbows, faint arcs just inside and near the top of the primary bow can be seen. These are called supernumerary arcs and were explained by Thomas Young in 1804 as arising from the interference of light along certain rays within the drop. Young's work had a profound influence on theories of the physical nature of light and his studies of the rainbow were a fundamental element of this. Young interpreted light in terms of it being a wave of some sort and that when two rays are scattered in the same direction within a raindrop, they may interfere with each other. Depending on how the rays mesh together, the interference can be constructive, in which case the rays produce a brightening, or destructive, in which case there is a reduction in brightness. This phenomenon is clearly described in Nussenzveig's article "The Theory of the Rainbow" in which he writes: "At angles very close to the rainbow angle the two paths through the droplet differ only slightly, and so the two rays interfere constructively. As the angle increases, the two rays follow paths of substantially different lengths. When the difference equals half of the wavelength, the interference is completely destructive; at still greater angles the beams reinforce again. The result is a periodic variation in the intensity of the scattered light, a series of alternately bright and dark bands."

Mikolaj and Pawel Sawicki have posted several beautiful photographs of rainbows showing these arcs.



The "purity" of the colors of the rainbow depends on the size of the raindrops. Large drops (diameters of a few millimeters) give bright rainbows with well defined colors; small droplets (diameters of about 0.01 mm) produce rainbows of overlapping colors that appear nearly white. And remember that the models that predict a rainbow arc all assume spherical shapes for raindrops.



There is never a single size for water drops in rain but a mixture of many sizes and shapes. This results in a composite rainbow. Raindrops generally don't "grow" to radii larger than about 0.5 cm without breaking up because of collisions with other raindrops, although occasionally drops a few millimeters larger in radius have been observed when there are very few drops (and so few collisions between the drops) in a rainstorm. Bill Livingston suggests: " If you are brave enough, look up during a thunder shower at the falling drops. Some may hit your eye (or glasses), but this is not fatal. You will actually see that the drops are distorted and are oscillating."



It is the surface tension of water that moulds raindrops into spherical shapes, if no other forces are acting on them. But as a drop falls in the air, the 'drag' causes a distortion in its shape, making it somewhat flattened. Deviations from a spherical shape have been measured by suspending drops in the air stream of a vertical wind tunnel (Pruppacher and Beard, 1970, and Pruppacher and Pitter, 1971). Small drops of radius less than 140 microns (0.014 cm) remain spherical, but as the size of the drop increases, the flattening becomes noticeable. For drops with a radius near 0.14 cm, the height/width ratio is 0.85. This flattening increases for larger drops.



Spherical drops produce symmetrical rainbows, but rainbows seen when the sun is near the horizon are often observed to be brighter at their sides, the vertical part, than at their top. Alistair Fraser has explained this phenomenon as resulting from the complex mixture of size and shape of the raindrops. The reflection and refraction of light from a flattened water droplet is not symmetrical. For a flattened drop, some of the rainbow ray is lost at top and bottom of the drop. Therefore, we see the rays from these flattened drops only as we view them horizontally; thus the rainbow produced by the large drops is is bright at its base. Near the top of the arc only small spherical drops produce the fainter rainbow.





What does a rainbow look like through dark glasses?

This is a "trick" question because the answer depends on whether or not your glasses are Polaroid. When light is reflected at certain angles it becomes polarized (discussed again quite well in Nussenzveig's article), and it has been found that the rainbow angle is close to that angle of reflection at which incident, unpolarized light (sunlight) is almost completely polarized. So if you look at a rainbow with Polaroid sunglasses and rotate the lenses around the line of sight, part of the rainbow will disappear!

Other Questions about the Rainbow

Humphreys (Physics of the Air, p. 478) discusses several "popular" questions about the rainbow:

"What is the rainbow's distance?" It is nearby or far away, according to where the raindrops are, extending from the closest to the farthest illuminated drops along the elements of the rainbow cone.

Why is the rainbow so frequently seen during summer and so seldom during winter?" To see a rainbow, one has to have rain and sunshine. In the winter, water droplets freeze into ice particles that do not produce a rainbow but scatter light in other very interesting patterns.

"Why are rainbows so rarely seen at noon?" Remember that the center of the rainbow's circle is opposite the sun so that it is as far below the level of the observer as the sun is above it.

"Do two people ever see the same rainbow?" Humphreys points out that "since the rainbow is a special distribution of colors (produced in a particular way) with reference to a definite point - the eye of the observer - and as no single distribution can be the same for two separate points, it follows that two observers do not, and cannot, see the same rainbow." In fact, each eye sees its own rainbow!!

Of course, a camera lens will record an image of a rainbow which can then be seen my many people! (thanks to Tom and Rachel Ludovise for pointing this out!)

"Can the same rainbow be seen by reflection as seen directly?" On the basis of the arguments given in the preceding question, bows appropriate for two different points are produced by different drops; hence, a bow seen by reflection is not the same as the one seen directly".

What are Reflection Rainbows?

A reflection rainbow is defined as one produced by the reflection of the source of incident light (usually the sun). Photographs of them are perhaps the most impressive of rainbow photographs. The reflected rainbow may be considered as a combination of two rainbows produced by sunlight coming from two different directions - one directly from the sun, the other from the reflected image of the sun. The angles are quite different and therefore the elevation of the rainbow arcs will be correspondingly different. This is illustrated in a diagram adapted from Greenler"s Rainbows, Halos, and Glories. The rainbow produced by sunlight reflected from the water is higher in the sky than is the one produced by direct sunlight.

What is a Lunar Rainbow?

A full moon is bright enough to have its light refracted by raindrops just as is the case for the sun. Moonlight is much fainter, of course, so the lunar rainbow is not nearly as bright as one produced by sunlight. Lunar rainbows have infrequently been observed since the time of Aristotle or before. A graphic description of one was writen by Dr. Mikkelson.

Rainbows and Proverbs

There is a delightful book by Humphreys entitled Weather Proverbs and Paradoxes. In it, he discusses the meteorological justifications of some proverbs associated with rainbows, such as "Rainbow at night, shepherd's delight;Rainbow in morning, shepherds take warning,"If there be a rainbow in the eve,It will rain and leave; But if there be a rainbow in the morrow It will neither lend nor borrow", and Rainbow to windward, foul fall the day; Rainbow to leeward, damp runs away."

The meteorological discussion Humphreys presents is appropriate for the northern temperate zones that have a prevailing wind, and also for a normal diurnal change in the weather.



Experiments

William Livingston, a solar astronomer who has also specialized in atmospheric optical phenomena suggests the following: "Try a hose spray yourself. As you produce a fine spray supernumeraries up to order three become nicely visible. "Try to estimate the size of these drops compared to a raindrop. ..."Another thing to try. View a water droplet on a leaf close-up - an inch from your eye. At the rainbow angle you may catch a nice bit of color!"

In Minnaert's excellent book Light and Colour in the Open Air you can find a number of experiments on how to study the nature of rainbows. Here is an illustration of one of his suggestions. Other demonstration projects are listed here .



Meg Beal, while a seventh-grader, prepared a science fair project that illustrated the nature of rainbows. The Beal family provided a photograph (1MB) of her excellent demonstration.



For those wanting to try to demonstrate the nature of a rainbow in a classroom, here are examples.



An informative tutorial on optics can be found here.



I am indebted to William C. Livingston, astronomer at the National Optical Astronomy Observatory in Tucson Arizona for his expert assistance in preparing this paper, and to Seth Sharpless for his critical reading of the manuscript. Charles A. Knight, an expert on rain at the National Center for Atmospheric Physics, provided valuable guidance on the interesting properties of raindrops.