P&R Labpak - Everything for your laboratory

P&R Labpak - Everything for your laboratory
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Wednesday, 19 December 2012

The Science of Snowflakes...


Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (−0 °F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice, then the droplet freezes around this "nucleus." Experiments show that this "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F). In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. The particles that make ice nuclei are very rare compared to nuclei upon which liquid cloud droplets form, however it is not understood what makes them efficient. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.


A snowflake often exhibits six-fold radial symmetry. The initial symmetry can occur because the crystalline structure of ice is six-fold. The six "arms" of the snowflake, or dendrites, then grow independently, and each side of each arm grows independently. Most snowflakes are not completely symmetric. The micro-environment in which the snowflake grows changes dynamically as the snowflake falls through the cloud, and tiny changes in temperature and humidity affect the way in which water molecules attach to the snowflake. Since the micro-environment (and its changes) are very nearly identical around the snowflake, each arm can grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grows the same; indeed, for some crystal forms it does not because the underlying crystal growth mechanism also affects how fast each surface region of a crystal grow.


Snowflakes form in a wide variety of intricate shapes, leading to the popular expression that "no two are alike". Although statistically possible, it is very unlikely for any two snowflakes to appear exactly alike. Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.

Friday, 14 December 2012

Why does salt melt ice?

­If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice.­ You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.
 Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C).

On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Two things happen when ice and water are placed in contact:
  • Molecules on the surface of the ice escape into the water (melting), and
  • molecules of water are captured on the surface of the ice (freezing).
When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average (although there are short-term fluctuations at the surface of the ice). The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0°C, the melting point of water, unless conditions change in a way that favours one of the processes over the other.

The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting.

Conversely, heating the mixture makes the molecules move faster on average, and melting is favoured.  Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. This leads to fewer water molecules on the liquid side because the some of the water has been replaced by salt. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing. That's why salt melts ice.

For more information see

Friday, 7 December 2012

It's a Cracker!!!

Silver fulminate (AgCNO) is the highly explosive silver salt of fulminic acid.
Silver fulminate is a primary explosive that has very little practical value due to its extreme sensitivity to impact, heat, pressure and electricity. The compound becomes progressively sensitive as it is aggregated, even in small amounts; the touch of a falling feather, the impact of a single water droplet or a small static discharge are all capable of explosively detonating an unconfined pile of silver fulminate no larger than a penny and no heavier than a few milligrams. Aggregation of larger quantities is impossible due to the compound's tendency to self-detonate under its own weight.

Silver fulminate was first prepared in 1800 by Edward Charles Howard in his research project to prepare a large variety of fulminates. Since its discovery, its only practical usage has been in producing non-damaging novelty noisemakers as children's toys and tricks - and Cracker snaps!

Silver fulminate, often in combination with potassium chlorate, is used in trick noise-makers known as "crackers", "snappers", "whippersnappers", "pop-its", or "bang-snaps", a popular type of novelty firework. They contain approximately 200 milligrams of fine gravel impregnated with a minute quantity (approximately 80 micrograms) of silver fulminate. When thrown against a hard surface, the impact is sufficient to detonate the tiny quantity of explosive, creating a small report from the supersonic detonation. Snaps are designed to be incapable of producing damage (even when detonated against skin) due to the buffering effect provided by the much greater mass of the gravel medium. It is also the chemical found in Christmas crackers. The chemical is painted on one of two narrow strips of card, with abrasive on the second. When the cracker is pulled the abrasive detonates the silver fulminate.

Remember this next time you pull a cracker!!!!!

Friday, 30 November 2012

The Amazing Spider Web!!!

A spider web, or cobweb (from the obsolete word coppe, meaning "spider") is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets.

Spider webs have existed for at least 141 million years. Insects get trapped in spider webs, providing nutrition to the spider; however, not all spiders build webs to catch prey, and some do not build webs at all. "Spider web" is typically used to refer to a web that is apparently still in use (i.e. clean), whereas "cobweb" refers to abandoned (i.e. dusty) webs.

Spiral orb webs in Karijini, Western Australia

Most spiders have three pairs of spinnerets, each having its own function – there are also spiders with just one pair and others with as many as four pairs.
Webs allow a spider to catch prey without having to expend energy by running it down. Thus it is an efficient method of gathering food. However, constructing the web is in itself an energetically costly process because of the large amount of protein required, in the form of silk. In addition, after a time the silk will lose its stickiness and thus become inefficient at capturing prey. It is common for spiders to eat their own web daily to recoup some of the energy used in spinning. The silk proteins are thus recycled.

The tensile strength of spider silk is greater than the same weight of steel and has much greater elasticity. Its microstructure is under investigation for potential applications in industry, including bullet-proof vests and artificial tendons.

Spider web covered in hoar frost
Types of spider webs
There are a few types of spider webs found in the wild, and many spiders are classified by the webs they weave. Different types of spider webs include:

  • Spiral orb webs, associated primarily with the family Araneidae, as well as Tetragnathidae and Uloboridae
  • Tangle webs or cobwebs, associated with the family Theridiidae
  • Funnel webs, with associations divided into primitive and modern
  • Tubular webs, which run up the bases of trees or along the ground
  • Sheet webs
Several different types of silk may be used in web construction, including a "sticky" capture silk and "fluffy" capture silk, depending on the type of spider. Webs may be in a vertical plane (most orb webs), a horizontal plane (sheet webs), or at any angle in between. It is hypothesized that these types of aerial webs co-evolved with the evolution of winged insects. As insects are spiders' main prey, it is likely that they would impose strong selectional forces on the foraging behavior of spiders. Most commonly found in the sheet-web spider families, some webs will have loose, irregular tangles of silk above them. These tangled obstacle courses serve to disorient and knock down flying insects, making them more vulnerable to being trapped on the web below. They may also help to protect the spider from predators such as birds and wasps.

The stickiness of spiders' webs is courtesy of droplets of glue suspended on the silk threads. This glue is multifunctional – that is, its behaviour depends on how quickly something touching it attempts to withdraw. At high velocities, they function as an elastic solid, resembling rubber; at lower velocities, they simply act as a sticky glue. This allows them to retain a grip on attached food particles.

Administering certain drugs to spiders affects the structure of the webs they build. It has been proposed by some that this could be used as a method of documenting and measuring the toxicity of various substances.  Visit the following link for more information.

Web created exposed to Caffeine


For more information on the wonders of webs....


Friday, 23 November 2012

PTFE - The most slippery substance in the world!!

PTFE - The Most Slippery Substance in the World

PTFE is the abbreviation for polytetrafluoroethene, a saturated fluorocarbon polymer, which was discovered serendipitously by Roy Plunkett, a 27-year-old research chemist working at the Du Pont Research Laboratories in Deepwater, New Jersey in 1938. He was actually doing some work for Kinetic Chemicals, a company founded jointly by Du Pont and General Motors to commercialise chlorinated fluorocarbon (CFC) refrigerants.

On the morning of 6 April of that year he went to use some tetrafluoroethene (TFE), a gas which was stored in a cylinder, which he needed to react with hydrogen chloride. The idea was that this would react with the C=C bond in TFE thus providing a route to hydrochlorofluorocarbon (HCFC) manufacture. To his astonishment, the cylinder which should have been holding 1000 grams of the gas only released 990 grams. Plunkett decided to dismantle the cylinder and, upon tipping it upside down, out came about 10 grams of a white waxy powder. Plunkett recorded in his laboratory notebook,

On cleaning up a cylinder which had contained approximately 1 kilo of tetrafluoroethylene, a white solid material was obtained, which was supposed to be a polymerised sample of C2F2...Sample gave good Beilstein test for halogen.

Thus Plunkett had realised that the gas had polymerised to form a new polymer, polytetrafluoroethene.

Further investigation of this polymer revealed that it had some remarkable properties: it was not attacked by corrosive acids, even if they were hot; it did not dissolve in solvents; it could be cooled to -240°C without becoming stiff and brittle, and it could be heated to 260°C without impairing its performance. Furthermore, it could be heated to over 500°C without burning or charring. In fact, PTFE is attacked only by molten sodium or fluorine gas under pressure; and so it rivals the noble metals, gold and platinum, in its unreactivity. Plunkett also noted that the substance had a slippery feel, and herein lay the secret of its later commercial success.

The difficulties of working with and characterising such an unreactive material were such that the development of processes for production of PTFE were prohibitively expensive, and Du Pont all but gave up on it. Indeed, unlike other plastics, it cannot be extruded, thermoformed, injection-moulded or rotomoulded. To 'work' it, techniques adapted from powder metallurgy must be used.

World War II and The Manhattan Project
This all changed in 1941 when the United States became embroiled in World War II and work on atomic bombs (the Manhattan Project) acquired a renewed urgency. Large quantities of fluorine were needed for the manufacture of uranium hexafluoride, from which the fissionable isotope 235U could be extracted in the gas-diffusion plant at Oak Ridge, Tennessee. However, both the fluorine and the uranium hexafluoride were so reactive that the provision of inert buffer gases, lubricants, coolants, gaskets, valve packings, reactor linings and pipes was critical. A saturated fluorocarbon, such as PTFE, would fit the bill perfectly. Teflon was also used for the diffusion membranes by which UF6 made from natural uranium could be isotopically enriched to 235UF6 for the atomic bomb.

During the war, a series of top-secret negotiations resulted in technical know-how and manufacturing rights being transferred to ICI (Plastics) in the United Kingdom. The ICI trade-name for PTFE is 'Fluon'.

Commercial Manufacture of PTFE
Du Pont gave the name 'Teflon' to its new polymer in 1945, and in 1950 they opened the world's first full commercial plant near Parkersburg, West Virginia.

The Non-Stick Frying Pan
The slippery feel of PTFE is due to the fact that it has extremely low intermolecular forces (van der Waals forces). The civil market for PTFE opened up in the 1950s once the American chemist, Louis Hartmann and the French engineer, Marc Grégoire independently discovered a way to bond PTFE to aluminium. This was achieved by treating the metal surface with acid, and applying the PTFE in emulsion form. The product is then baked at 400°C for a few minutes, allowing the polymer to melt and form a film over the surface. Following this, the Tefal company was set up in 1956 to market non-stick cookware.

In 1969, Dr Bob Gore found a way of expanding PTFE by heating and stretching it to form a membrane with microscopic pores in the structure. There are billions of these pores per square centimetre and they are small enough to keep water droplets out whilst allowing water molecules, present as vapour from sweat, out. Such a material is said to be 'microporous'. The PTFE membrane is sandwiched between the outer fabric and inner lining of the garment, whilst between the membrane and the inner lining is a layer of an oil-hating (lipophobic) polymer. This also allows the water vapour through, but prevents the natural oils of the skin from getting through and blocking the micropores in the PTFE. Gore-tex is now widely used in wet-weather gear and sportswear.

An interesting application of Gore-tex is as a biocompatible membrane to facilitate bone tissue restoration in patients with long-standing periodontal disease. In addition, it is possible to restore or regenerate bone prior to the placement of bridges or implants.

Other applications for Gore-tex in the bio-medical field include its use for artificial veins, arteries and trachea replacements. It is also used for artificial dentures and corneas (for example, for patients suffering from keratoconus) and as substitute bones for chin, nose, skull and hip.

Other applications of PTFE

Domestically, PTFE is used as a stain-repellent on clothes, furniture covers and carpets, where it is marketed under such trade-names as Scotchgard and Zepel. It is also used on the underside of electric irons and as dental floss.

PTFE is well-known as the plumbers' tape for sealing joints in central heating systems. It is also an excellent electrical insulator and is thus used in electrical wires and cables.

PTFE has the lowest coefficient of friction of any solid material, due to having very low intermolecular (van der Waals) forces. Hence it finds use as lubricant-free bearings in motors. Scrap PTFE from industry is re-used by grinding it to a micro-fine powder and adding it to printers' ink where it facilitates ink flow.

For more information visit



Friday, 16 November 2012

How does Soap Work?

Soap is a curious substance, designed to solve an intriguing problem. Most dirt that will not simply wipe off or be shaken out is in fact some form of fat or grease. In most households the most common cleaning agent is tap water. The problem is that grease and water fall into two different and largely incompatible chemical groups. Drop oil into water, and it will tend to float or form discrete droplets. Pour water into oil and you will see the same effect. Additionally, substances such as salt and sugar that dissolve in water will not dissolve in oil, whereas something like petrol will only float on water but is quite capable of dissolving oil

The Chemistry of Oils

This difference in behaviour is due to the nature of the molecules involved. Water is largely polar, that is, water molecules tend to separate into fragments with opposite electrical charges, one positive and one negative. Chemicals such as table salt that happen to be made up of collections of charged fragments, or ions, find it easy to dissolve in water because the positive ions in the salt are attracted to the negative ions in the water, and vice versa. Similarly, the charged nature of water means that water is a good conductor of electricity.

Fats and oils, on the other hand, tend not to be polar. Their molecules have no particular electrical charge, and so are not attracted to polar substances such as salt. Instead, they prefer to bond with other non-polar substances. Fats and oils tend to be electrical insulators.

Washing Up

This, then, returns us to the washing-up. You have a greasy dish in a bowl of water, but the grease is showing no inclination to dissolve in the water because the water is polar and the grease is not. Attack the grease with a cloth and most of what you achieve is to move it around on the plate, because it is trying to flatten itself against the surface of the plate in a effort to get away from the water molecules.

The soap molecule is a halfway house. It consists of a long strand with an ionic water-loving, grease-repelling group on one end, and a non-polar grease-loving, water-repelling group on the other. If you drop soap into clean water, all the molecules gather on the surface with their water-loving (hydrophilic) ionic ends stuck in the water and their fat-loving (lipophilic) ends waving in the air. Slide a dirty dish in, however, and the lipophilic end of each molecule sticks to the grease as it slips past. As the dish sinks, it takes the soap molecules with it, attached by their heads to the grease but still waving their hydrophilic tails in the water like microscopic tadpoles.

All you have to do now is bash at the dirt with a sponge or cloth, and it can be persuaded to leave the plate, for as it lifts off the surface it becomes insulated from the water as new soap molecules rush in and try to bury their heads in it. The end result is a small blob of grease completely surrounded by a layer of soap molecules, all with their lipophilic heads pointing inwards and their hydrophilic tails pointing outwards. As far as the grease is concerned, all it can see are lipophilic molecules, and as far as the water is concerned, all it can see is a rather large hydrophilic lump.

Eventually, of course, all the soap molecules are used up, and you have to tip out the washing-up water and start again. Pass the tea-towel.

For more information

Friday, 9 November 2012

Thunderbolts & Lightning...very very frightening?

Thunderstorms – An Introduction
Continuing on our weather theme from last week, we're looking at Thunder & Lightning today.

A thunderstorm can be described as one or more sudden electrical discharges, manifested by a flash of light (lightning) and a sharp or rumbling sound (thunder). Thunderstorms are associated with convective clouds and are most often, but not necessarily, accompanied by precipitation at the ground.

Cumulonimbus clouds (Latin: cumulus – heap; nimbus – rainy cloud)

A cumulonimbus ‘cloud factory’ (© J. Corey).

Not all cumulonimbus clouds bring thunderstorms; some just bring heavy showers or hail. On average, an individual cumulonimbus cloud takes only one hour to take shape, grow and dissipate. It produces less than 30 minutes of thunder and lightning. If a thunderstorm lasts longer than this, it is probably because there is more than one cumulonimbus present.

Electrical charges within a cumulonimbus cloud

Lightning is a large electrical spark caused by electrons moving from one place to another

Lightning seen over Iowa (© M. Clark).

Electrons are fundamental sub-atomic particles that carry a negative electric charge. They are so small they cannot be seen, but when lightning flashes they are moving so fast that the air around them glows. The actual streak of lightning is the path the electrons follow when they move.

Water droplets form inside a storm cloud. They are propelled towards the top of the cloud by strong internal winds (updraughts), where they turn to ice. Some of the pieces of ice grow large into hail, but others remain very small. As the pieces of hail get larger, they fall back through the cloud, bumping into smaller ice particles that are still being forced upwards. When the ice particles collide, some electrons are transferred to the hail. The electrons give the hail a negative charge, while the ice particles that have lost electrons gain a positive charge.

The winds continue to carry the ice particles upwards, giving the top of the cloud a positive charge. Some of the hail has now grown so heavy that the winds can no longer propel them upwards and so collect in the lower part of the cloud, giving it a negative charge. As well as being attracted to the positive atoms in the top of the cloud, the atoms are attracted to positive atoms in other clouds and on the ground. If the attraction is strong enough, the electrons will move towards the positive atoms. The path they make in doing so is the flash of lightning.

As negative charges collect at the base of the cloud, they repel the electrons near the ground’s surface. This leaves the ground and the objects on it with a positive charge. As the attraction between the cloud and the ground grows stronger, electrons shoot down from the cloud. The electrons move in a path that spreads in different directions — like a river delta. Each step is approximately 50 metres long and the branching path is called a stepped leader. Further electrons follow, making new branches. The average speed at which the stepped leader cuts through the air is about 270,000 miles per hour.

Types of lightning

There are several types of lightning, these are:

• Ball lightning — a rare form of lightning in which a persistent and moving luminous white or coloured sphere is seen.

• Rocket lightning — a very rare and unexplained form of lightning in which the speed of propagation of the lightning stroke is slow enough to be perceptible to the eye.

• Pearl-necklace lightning — a rare form of lightning, also termed ‘chain lightning’ or ‘beaded lightning’, in which variations of brightness along the discharge path give rise to a momentary appearance similar to pearls on a string.

• Ribbon lightning — ordinary cloud-to-ground lightning that appears to be spread horizontally into a ribbon of parallel luminous streaks when a very strong wind is blowing at right angles to the observer’s line of sight.

• Forked lightning — lightning in which many luminous branches from the main discharge channel are visible.

• Sheet lightning — the popular name applied to a ‘cloud discharge’ form of lightning in which the emitted light appears diffuse and there is an apparent absence of a main channel because of the obscuring effect of the cloud.

• Streak lightning — lightning discharge which has a distinct main channel, often tortuous and branching, the discharge may be from cloud to ground or from cloud to air.

Forked lightning (© M.J.O. Dutton).


The word ‘thunder’ is derived from ‘Thor’, the Norse god of thunder. He was supposed to be a red-bearded man of tremendous strength; his greatest attribute being the ability to forge thunderbolts. The word Thursday is also derived from his name.

Thunder is the sharp or rumbling sound that accompanies lightning. It is caused by the intense heating and expansion of the air along the path of the lightning. The rumble of thunder is caused by the noise passing through layers of the atmosphere at different temperatures. Thunder lasts longer than lightning because of the time it takes for the sound to travel from different parts of the flash.

How far away is a thunderstorm?

This can roughly be estimated by measuring the interval between the lightning flash and the start of the thunder. If you count the time in seconds and then divide by three, you will have the approximate distance in kilometres. Thunder is rarely heard at a distance of more than 20 km.

Are thunderstorms dangerous?

Most people are frightened by the crackles and rumbles of thunder rather than the flash of lightning. However, thunder cannot hurt anybody, and the risk of being struck by lightning is far less than that of being killed in a car crash. Ninety per cent of lightning discharges go from cloud to cloud or between parts of the same cloud, never actually reaching the Earth. Most of the discharges that do strike the ground cause little or no damage or harm. Lightning takes the shortest and quickest route to the ground, usually via a high object standing alone.

How common are thunderstorms?

Thunderstorms occur throughout the world, even in polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. The most thundery part of the earth is the island of Java where the annual frequency of thunderstorms is about 220 days per year. In temperate regions, they are most frequent in spring and summer, although they can occur in cold fronts at any time of year. Thunderstorms are rare in polar regions due to the cold climate and stable air masses that are generally in place, but they do occur from time to time, mainly in the summer months. In recent years, thunderstorms have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the visual and scientific aspects of storms and tornadoes.

In the United Kingdom thunder is a variable element, the highest and lowest annual totals of thunderstorm days at many individual stations ranges from less than 5 in a quiet year to 20 or more in an active one. One consequence of this is that published maps showing the average frequency of days of thunder differ considerably in detail according to the period of records used. They agree, however, in showing that the average annual frequency is less than 5 days in western coastal districts and over most of central and northern Scotland, and 15 to 20 days over the east Midlands and parts of southeast England. There is relatively little seasonal variation on the western seaboard but elsewhere summer is the most thundery season.
For more information on this subject visit www.metoffice.gov.uk

Friday, 2 November 2012

What is a Rainbow?

Double rainbow and supernumerary rainbows on the inside of the primary arc. The shadow of the photographer's head on the bottom marks the centre of the rainbow circle (antisolar point).

A rainbow is an optical and meteorological phenomenon that is caused by reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc.

Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

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 while 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, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.


The rainbow is not located at a specific distance, but comes from any water droplets viewed from a certain angle relative to the Sun's rays. Thus, a rainbow is not an object, and cannot be physically approached. Indeed, it is impossible for an observer to manoeuvre to see any rainbow from water droplets at any angle other than the customary one of 42 degrees from the direction opposite the Sun. Even if an observer sees another observer who seems "under" or "at the end" of a rainbow, the second observer will see a different rainbow further off-yet, at the same angle as seen by the first observer. A rainbow spans a continuous spectrum of colours. Any distinct bands perceived are an artefact of human colour vision, and no banding of any type is seen in a black-and-white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side. For colours seen by a normal human eye, the most commonly cited and remembered sequence is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet.

Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.

Rainbows can form in mist, such as that of a waterfall
Rainbow with a faint reflected rainbow in the lake
Rainbows may form in the spray created by waves (called spray bows)

Rainbow after sunlight bursts through after an intense shower in Maraetai, New Zealand
Circular rainbow seen while skydiving over Rochelle, Illinois
Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun. The result is a luminous rainbow that contrasts with the darkened background.
The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white. It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre.
Number of colours in spectrum or rainbow
A spectrum obtained using a glass prism and a point source, is a continuum of wavelengths without bands. The number of colours that the human eye is able to distinguish in a spectrum is in the order of 100. Accordingly, the Munsell colour system (a 20th century system for numerically describing colours, based on equal steps for human visual perception) distinguishes 100 hues. However, the human brain tends to divide them into a small number of primary colours. The apparent discreteness of primary colours is an artefact of the human brain. Newton originally (1672) divided the spectrum into five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven primary colours by analogy to the number of notes in a musical scale. The Munsell colour system removed orange and indigo again, and returned to five primary colours. The exact number of primary colours for humans is a somewhat arbitrary choice.

Light rays enter a raindrop from one direction (typically a straight line from the Sun), reflect off the back of the raindrop, and fan out as they leave the raindrop. The light leaving the rainbow is spread over a wide angle, with a maximum intensity at 40.89–42°
White light separates into different colours on entering the raindrop due to dispersion, causing red light to be refracted less than blue light.
Multiple rainbows
Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.
A double rainbow features reversed colours in the outer (secondary) bow, with the dark Alexander's band between the bows.

Monochrome rainbow
Occasionally a shower may happen at sunrise or sunset, where the shorter wavelengths like blue and green have been scattered and essentially removed from the spectrum. Further scattering may occur due to the rain, and the result can be the rare and dramatic monochrome rainbow.
Fogbows form in the same way as rainbows, but they are formed by much smaller cloud and fog droplets which diffract light extensively. They are almost white with faint reds on the outside and blues inside. The colours are dim because the bow in each colour is very broad and the colours overlap. Fogbows are commonly seen over water when air in contact with the cooler water is chilled, but they can be found anywhere if the fog is thin enough for the sun to shine through and the sun is fairly bright. They are very large—almost as big as a rainbow and much broader. They sometimes appear with a glory at the bow's centre
For more information on Rainbows visit one of the following:-