The color of water varies with the ambient conditions in which that water is present. While relatively small quantities of water appear to be colorless, pure water has a slight blue color that becomes deeper as the thickness of the observed sample increases. The hue of water is an intrinsic property and is caused by selective absorption and scattering of blue light. Dissolved elements or suspended impurities may give water a different color.
The intrinsic color of liquid water may be demonstrated by looking at a white light source through a long pipe that is filled with purified water and closed at both ends with a transparent window. The light cyan color is caused by weak absorption in the red part of the visible spectrum. [2]
Absorptions in the visible spectrum are usually attributed to excitations of electronic energy states in matter. Water is a simple three-atom molecule, H2O, and all its electronic absorptions occur in the ultraviolet region of the electromagnetic spectrum and are therefore not responsible for the color of water in the visible region of the spectrum. The water molecule has three fundamental modes of vibration. Two stretching vibrations of the O–H bonds in the gaseous state of water occur at v1 = 3650 cm−1 and v3 = 3755 cm−1. Absorption due to these vibrations occurs in the infrared region of the spectrum. The absorption in the visible spectrum is due mainly to the harmonic v1 + 3v3 = 14,318 cm−1, which is equivalent to a wavelength of 698 nm. In liquid state at 20 °C (68 °F) these vibrations are red-shifted by hydrogen bonding, resulting in red absorption at 740 nm, other harmonics such as v1 + v2 + 3v3 giving red absorption at 660 nm. [3] The absorption curve for heavy water (D2O) is of a similar shape, but is shifted further towards the infrared end of the spectrum, because the vibrational transitions have a lower energy. For this reason, heavy water does not absorb red light and thus large bodies of D2O would lack the characteristic cyan color of the more commonly found light water (1H2O). [4]
Absorption intensity decreases markedly with each successive overtone, resulting in very weak absorption for the third overtone. For this reason, the pipe needs to have a length of a meter or more and the water must be purified by microfiltration to remove any particles that could produce Mie scattering.
Lakes and oceans appear cyan for several reasons. One is that the surface of the water reflects the color of the sky, which ranges from cyan to light azure. It is a common misconception that this reflection is the sole reason bodies of water appear cyan, though it can contribute. This contribution usually makes the body of water appear more a shade of azure rather than cyan depending on how bright the sky is. [5] [6] Water in swimming pools with white-painted sides and bottom will appear cyan, even in indoor pools where there is no sky to be reflected. The deeper the pool, the more intense the cyan color becomes. [7]
Some of the light hitting the surface of the ocean is reflected but most of it penetrates the water surface, interacting with water molecules and other substances in the water. Water molecules can vibrate in three different modes when they interact with light. The red, orange, and yellow wavelengths of light are absorbed so the remaining light seen is composed of green, cyan, and blue wavelengths. This is the main reason the ocean's color is cyan. The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle. [8]
Scattering from suspended particles also plays an important role in the color of lakes and oceans, causing the water to look greener or bluer in different areas. A few tens of meters of water will absorb all light, so without scattering, all bodies of water would appear black. Because most lakes and oceans contain suspended living matter and mineral particles, light from above is scattered and some of it is reflected upwards. Scattering from suspended particles would normally give a white color, as with snow, but because the light first passes through many meters of cyan-colored liquid, the scattered light appears cyan. In extremely pure water—as is found in mountain lakes, where scattering from particles is very low—the scattering from water molecules themselves also contributes a cyan color. [9] [10]
Diffuse sky radiation due to Rayleigh scattering in the atmosphere along one's line of sight gives distant objects a cyan or light azure tint. This is most commonly noticed with distant mountains, but also contributes to the cyanness of the ocean in the distance.[ citation needed ]
Glaciers are large bodies of ice and snow formed in cold climates by processes involving the compaction of fallen snow. While snowy glaciers appear white from a distance, the long path lengths of internal reflected light causes glaciers to appear a deep blue when viewed up close and when shielded from direct ambient light.[ citation needed ]
Relatively small amounts of regular ice appear white because plenty of air bubbles are present, and also because small quantities of water appear to be colorless. In glaciers, on the other hand, the pressure causes the air bubbles, trapped in the accumulated snow, to be squeezed out increasing the density of the created ice. Large quantities of water appear cyan, therefore a large piece of compressed ice, or a glacier, would also appear cyan.
Dissolved and particulate material in water can cause it to be appear more green, tan, brown, or red. For instance, dissolved organic compounds called tannins can result in dark brown colors, or algae floating in the water (particles) can impart a green color. [11] Color variations can be measured with reference to a standard color scale. Two examples of standard color scales for natural water bodies are the Forel-Ule scale and the Platinum-Cobalt scale. For example, slight discoloration is measured against the Platinum-Cobalt scale in Hazen units (HU). [12]
The color of a water sample can be reported as:
Testing for color can be a quick and easy test which often reflects the amount of organic material in the water, although certain inorganic components like iron or manganese can also impart color.
Water color can reveal physical, chemical and bacteriological conditions. In drinking water, green can indicate copper leaching from copper plumbing and can also represent algae growth. Blue can also indicate copper, or might be caused by syphoning of industrial cleaners in the tank of commodes, commonly known as backflowing. Reds can be signs of rust from iron pipes or airborne bacteria from lakes, etc. Black water can indicate growth of sulfur-reducing bacteria inside a hot water tank set to too low a temperature. This usually has a strong sulfur or rotten egg (H2S) odor and is easily corrected by draining the water heater and increasing the temperature to 49 °C (120 °F) or higher. Sulfate reducing bacteria are not known to cause issues in cold water plumbing.[ citation needed ] Learning the water impurity indication color spectrum can make identifying and solving cosmetic, bacteriological and chemical problems easier.
The presence of color in water does not necessarily indicate that the water is not drinkable. Water with high water clarity is generally more cyan in color due to low concentrations of particles and/or dissolved substances. Color-causing particulate substances can be easily removed by filtration. Color-causing dissolved substances such as tannins are only toxic to animals in large concentration. [14]
Color from dissolved substances is not removed by typical water filters; however the use of coagulants may succeed in trapping the color-causing compounds within the resulting precipitate.[ citation needed ] Other factors can affect the color seen:
Various cultures divide the semantic field of colors differently from the English language usage, and some do not distinguish between blue and green in the same way. An example is Welsh in which glas can mean blue or green, or Vietnamese in which xanh likewise can mean either. Conversely, in Russian and some other languages, there is no single word for blue, but somewhat different words for light blue (голубой, goluboy) and dark blue (синий, siniy).
Other color names assigned to bodies of water are sea green and ultramarine blue. Unusual oceanic colorings have given rise to the terms red tide and black tide.
The Ancient Greek poet Homer uses the epithet "wine-dark sea"; in addition, he also describes the sea as "grey". William Ewart Gladstone has suggested that this is due to the Ancient Greek classifying colors primarily by luminosity rather than hue, while others believe Homer was color blind.[ citation needed ]
The Ancient Indian wisdom of Veda considers water's life-giving contributions a part of the divine. It recognizes water as an ancient god, Varuna, and the color of Varuna is described as blue. In the Gayatri associated with Varuna, the phrase "Neela purusha" comes in the second line, which calls the water deity the blue one.
Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.
Cyan is the color between blue and green on the visible spectrum of light. It is evoked by light with a predominant wavelength between 500 and 520 nm, between the wavelengths of green and blue.
Fluorescence is one of two kinds of emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colored visible light. The color of the light emitted depends on the chemical composition of the substance. Fluorescent materials generally cease to glow nearly immediately when the radiation source stops. This distinguishes them from the other type of light emission, phosphorescence. Phosphorescent materials continue to emit light for some time after the radiation stops.
Rayleigh scattering is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering medium, the amount of scattering is inversely proportional to the fourth power of the wavelength. The phenomenon is named after the 19th-century British physicist Lord Rayleigh.
Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
In physics, attenuation is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable attenuation rates.
The photic zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological processes that supply nutrients into the upper water column. The photic zone is home to the majority of aquatic life due to the activity of the phytoplankton. The thicknesses of the photic and euphotic zones vary with the intensity of sunlight as a function of season and latitude and with the degree of water turbidity. The bottommost, or aphotic, zone is the region of perpetual darkness that lies beneath the photic zone and includes most of the ocean waters.
The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to an absorption of energy performed by the chemical.
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to electrons making a transition from a high energy state to a lower energy state. The photon energy of the emitted photons is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.
In the field of optics, transparency is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale, the photons can be said to follow Snell's law. Translucency allows light to pass through but does not necessarily follow Snell's law; the photons can be scattered at either of the two interfaces, or internally, where there is a change in the index of refraction. In other words, a translucent material is made up of components with different indices of refraction. A transparent material is made up of components with a uniform index of refraction. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. The opposite property of translucency is opacity. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.
The Tyndall effect is light scattering by particles in a colloid such as a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, in particular two-stroke machines where the burnt engine oil provides these particles. The same effect can also be observed with tobacco smoke whose fine particles also preferentially scatter blue light.
Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of the glacier. During compression, air bubbles are squeezed out, so ice crystals enlarge. This enlargement is responsible for the ice's blue colour.
Underwater vision is the ability to see objects underwater, and this is significantly affected by several factors. Underwater, objects are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water. They are also blurred by scattering of light between the object and the viewer, also resulting in lower contrast. These effects vary with wavelength of the light, and color and turbidity of the water. The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact. Provision of an airspace between the cornea and the water can compensate, but has the side effect of scale and distance distortion. The diver learns to compensate for these distortions. Artificial illumination is effective to improve illumination at short range.
Ocean color is the branch of ocean optics that specifically studies the color of the water and information that can be gained from looking at variations in color. The color of the ocean, while mainly blue, actually varies from blue to green or even yellow, brown or red in some cases. This field of study developed alongside water remote sensing, so it is focused mainly on how color is measured by instruments.
The absorption of electromagnetic radiation by water depends on the state of the water.
Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification. Highly turbid ocean waters are those with many scattering particulates in them. In both highly absorbing and highly scattering waters, visibility into the water is reduced. Highly scattering (turbid) water still reflects much light, while highly absorbing water, such as a blackwater river or lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including sediments and phytoplankton.
Atmospheric optics is "the study of the optical characteristics of the atmosphere or products of atmospheric processes .... [including] temporal and spatial resolutions beyond those discernible with the naked eye". Meteorological optics is "that part of atmospheric optics concerned with the study of patterns observable with the naked eye". Nevertheless, the two terms are sometimes used interchangeably.
Water Remote Sensing is the observation of water bodies such as lakes, oceans, and rivers from a distance in order to describe their color, state of ecosystem health, and productivity. Water remote sensing studies the color of water through the observation of the spectrum of water leaving radiance. From the spectrum of color coming from the water, the concentration of optically active components of the upper layer of the water body can be estimated via specific algorithms. Water quality monitoring by remote sensing and close-range instruments has obtained considerable attention since the founding of EU Water Framework Directive.
Chappuis absorption refers to the absorption of electromagnetic radiation by ozone, which is especially noticeable in the ozone layer, which absorbs a small part of sunlight in the visible portion of the electromagnetic spectrum. The Chappuis absorption bands occur at wavelengths between 400 and 650 nm. Within this range are two absorption maxima of similar height at 575 and 603 nm.
Ocean optics is the study of how light interacts with water and the materials in water. Although research often focuses on the sea, the field broadly includes rivers, lakes, inland waters, coastal waters, and large ocean basins. How light acts in water is critical to how ecosystems function underwater. Knowledge of ocean optics is needed in aquatic remote sensing research in order to understand what information can be extracted from the color of the water as it appears from satellite sensors in space. The color of the water as seen by satellites is known as ocean color. While ocean color is a key theme of ocean optics, optics is a broader term that also includes the development of underwater sensors using optical methods to study much more than just color, including ocean chemistry, particle size, imaging of microscopic plants and animals, and more.
Heavy water is colourless because all of its corresponding vibrational transitions are shifted to lower energy (higher wavelength) by the increase in isotope mass.
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