Absorbent -
The process in which matter adheres to the surface of an absorbant.
Acid -
A substance which releases hydrogen ions when dissolved in water.Most acids will dissolves the common metals, and will react with a base to form a neutral salt and water.
Activated Carbon -
A granular material usually produced by the roasting of cellulose base substances, such as wood or coconut shells, in the absence of air.It has a very porous structure and is used in water conditioning as an absorbent for organic matter and certain dissolved gases. sometimes called " activated charcoal ".
Alkalinity -
The quantitative capacity of a water or water solution to neutralize an acid. It is usually measured by titration with a standard acid solution of sulfuric acid, and expressed in terms of its calcium carbonate equivalent.
Anion -
A negatively charged ion in solution, such as bicarbonate, chloride, or sulfate.
Anion Exchange -
An ion exchange process in which anions in solution are exchanged for other anions for an ion exchanger. In demineralization, for example, bicarbonate, chloride and sulfate anions are removed from solution in exchange for a chemically equivalent number hydroxide anions from the anion exchange resin.
Aquifier -
A layer or zone below the surface of the earth which is capable of yielding a significant volume of water.
Atom -
The smallest particle of an element that can exist either alone or in combination with similar particles of the same element or of a different element.
Attrition -
The process in which solids are worn down or ground down by friction, often between particles of the same material. filter media and ion exchange materials are subject to attrition during backwashing, regeneration, and service.
Backwash -
The process in which beds of filter or ion exchange media are subjected to flow opposite to the service flow direction to loosen the bed and to flush suspended matter, collected during the service run, to waste.
Bacteria -
Unicellular micro-organisms which typically reproduce by cell division. Although usually classed as plants, bacteria contain no chlorophyll.
Base -
A substance which releases hydroxyl ions when dissolved in water. Bases react with acids to form a neutral salt and water.
Bed -
the ion exchange or filter media in a column or other tank or operational vessel.
Bed Depth -
the height of the ion exchange or filter media in the vessel after preparation for service
Bicarbonate Alkalinity -
he alkalinity of a water due to the presence of bicarbonate ions.
Biochemical Oxygen Demand -
the amount of oxygen consumed in the oxidation of organic matter by biological action under specific standard test conditions. Widely used as a measure of the strength of sewage and waste water.
Brine -
a strong solution of salts, such as the sodium chloride brine used in the regeneration of ion exchange water softeners, but also applied to the mixed sodium, calcium and magnesium chloride waste solution from regeneration.
Calcium -
one of the principal elements making up the earths crust, the compounds of which make the water hard. The presence of calcium in water is a factor contributing to the formation of scale and insoluble soap curds which are a means of clearly identifying hard water.
Capacity -
an expression of the quantity of an undesirable material which can be removed by a water conditioner between servicing of the media, i.e., cleaning regeneration or replacement, as determined under standard test conditions. For ion exchange water softeners, the capacity is expressed in grains of hardness removal between successive regenerations and is related to the pound of salt used in regeneration. For filters, the capacity may be expressed in the length of time or total gallons delivered between servicing.
Carbon Dioxide -
a gas present in the atmosphere and formed by the decay of organic matter;the gas in carbonated beverages; in water it forms carbonic acid.
Carbonate -
the Co3- ion.
Carbonate Alkalinity -
alkalinity due to the presence of the carbonate ion.
Carbonate Hardness -
hardness due to the presence of calcium and magnesium bicarbonates and carbonates in water; the smaller of the total hardness and the total alkalinity.
Cation -
an ion with a positive electrical charge, such as calcium, magnesium and sodium.
Cation Exchange -
process in which cations in solution are exchanged for other cations from an ion exchanger.
Caustic -
Any substance capable of burning or destroying animal flesh or tissue. The term is applied to strong bases.
Caustic Soda -
the common name sodium hydroxide.
Chelating Agent -
a chemical compound sometimes fed to water to tie up undesirable metal ions, keep them in solution, and eliminate or reduce the normal effects of the ion.
Chemical Oxygen Demand -
the amount of matter, both organic and inorganic, in a water or wastewater which can be oxidized by boiling with a strong oxidizing acid under standard test conditions, and expressed as the equivalent amount of oxygen; often used as a measure of the strength of sewage and wastewater; includes materials not oxidized in the BOD test, and thus does not correlate with BOD.
Chlorine -
a gas, widely used in the disinfection of water and an oxidizing agent for organic matter, iron, etc.
Chlorine Demand -
a measure of the amount of chlorine which will be consumed by organic matter and other oxidizable substances in a water before a chlorine residual will be found; the difference between the total chlorine fed and the chlorine residual.
Coagulant -
a material, such as alum, which will form a gelatinous precipitate in water, and cause the agglomeration of finely divided particles into larger particles which can then be removed by settling and/or filtration.
Coagulant Aid -
a material which is not a coagulant , but which improves the effectiveness of a coagulant, often by forming a larger or heavier particles, speeding the reactions, or by permitting reduced coagulant dosage.
Coagulation -
The process in which very small, finely divided solid particles, often colloidal in nature, are agglomerated into larger particles.
Colloid -
very finely divided solid particles which will not settle out of a solution; intermediate between a true dissolved particle and a suspended solid which will settle out of solution. the removal colloidal particles usually requires coagulation to form larger particles which may be removed by sedimentation and/or filtration.
Compensated Hardness -
a calculated value based on the total hardness, the magnesium to calcium ratio and the sodium concentration of a water. it is used to correct for the reductions in hardness removal capacity caused by these factors in cation exchange water softeners. No single method of calculation has been widely accepted.
Conductance -
a measure of the ability of a solution to carry electricity, the reciprocal of the electrical resistance. The unit of conductance is the mho(reciprocal ohm).
Conductivity -
the quality or power to carry electrical current; in water, the conductivity is related to the concentration of ions capable of carrying electrical current.
Corrosion -
the destructive disintegration of a metal by electrochemical means.
Cycle -
a series of events or steps which ultimately lead back to the starting point, such as the exhaustion-regeneration cycle of an ion exchange system; sometimes incorrectly used in reference to a single step to a complete cycle.
D.I. -
abbreviation for "deionization"
Deionization -
the removal of all ionized minerals and salts (both organic and inorganic) from a solution by a two stage ion exchange procedure. First, positively charged for a chemically equivalent amount of hydrogen ions. Second, negatively charged ions are removed by an ion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions are introduced in this process unite to form water molecules. The term is often used interchangeably with demineralization.
Demineralization -
the removal of ionized inorganic minerals and salts (not organic materials)from a solution by a two phase ion exchange procedure, similar to deionization, and the two terms are often used interchangeably.
Dialysis -
the separation of components of a solution by diffusion through a semi-permeable membrane which is capable of passing certain ions or molecules while rejecting others.
Differential Pressure -
the difference in pressure at two points in a water system, may be due to differences in elevation or to friction losses or pressure drops due to resistance to flow in pipes, softeners, filters or other devices.
Disinfection -
a process in which pathogenic (disease producing bacteria) are killed; may involve disinfecting agents such as chlorine, or physical processes such as heating.
Dissolved Solids -
the weight of matter in true solution in a stated volume of water; includes both inorganic and organic matter; usually determined by weighing the residue after evaporation of the water at 105 or180c.
Distillation -
the process in which a liquid, such as water, is converted into its vapor state and collected; used to remove solids and other impurities from water; multiple distillations are required for extreme purity.
Efficiency -
the ratio of output per unit input; the effectiveness of performance of a system; in an ion exchange system, often expressed as the amount of regenerant required to produce a unit of capacity, such as the pounds of salt per kilogram of hardness removal.
Electrodialis -
a process in which a direct current is applied to a cell to draw charged ions through ion selective semipermiable membranes, thus removing the ions from the solution.
Endpoint -
the point at which a process is stopped because a predetermined value of a measurable variable is reached, the endpoint of an ion exchanger water softener service run is the point at which the hardness of the softener effluent increases to a predefined concentration, often 1.0 grain per gallon; the endpoint of a filter service run may be the point at which the pressure drop across the filter reaches a predetermined value; the endpoint of a titration is the point at which the titrant produces a predetermined color change, ph value, or other measurable characteristic.
Equivalent Per Million -
a unit of concentration used in chemical calculations, calculated by dividing the concentration in ppm or mg/1 by the equivalent weight.
Exhaustion -
the state of an ion exchange material in which it is no longer capable of effective function due to the depletion of the initial supply of exchangeable ions; the exhaustion point may be defined in terms of a limiting concentration of matter in the effluent, or in the case of demineralization, in terms of electrical conductivity.
Filter -
specifically, a device or system for the removal of solid particles(suspended solids); in general, includes mechanical, adsorptive, oxidizing, and neutralizing filters.
Fixture Unit -
an arbitrary unit assigned to different types of plumbing fixtures, and used to estimate flow rate requirements and drain capacity requirements.
Flocculation -
the agglomeration of finely divided suspended solids into larger, usually gelatinous, particles; the development of a "floc" after treatment with a coagulant by gentle stirring or mixing.
Flow Control -
a device designed to limit the flow of water of regenerant to a predetermined value over a broad range of inlet water pressures.
Flow Rate -
the quantity of water a regenerant which passes a given point in a specified unit of time, often expressed in gallons per minute.
Fluoridation -
the addition of a fluoride compound to a water supply to produce the concentration desired for the reduction in incidence of dental carries.
Flush Tank -
a tank or chamber in which water is stored for pid release to flush a toilet or water closet.
Flush Valve -
a self closing valve designed to release a large volume of water when tripped.
Fouling -
the process in which undesirable foreign matter accumulates in a bed of filter media or ion exchanger, clogging pores and coating surfaces and thus inhibiting or retarding the proper operation of the bed.
Free Available Chlorine -
the concentration of residual chlorine present as dissolved gas, htpochlorous acid or hypochlorite, not combined with ammonia or in other less available form.
Freeboard -
the vertical distance between a bed of filter media or ion exchange material and the overflow or collector for backwash water; the height above the bed of granular media available for bed expansion during backwashing; may be expressed either as a linear distance or a percentage of bed depth.
GPG -
abbreviation for 'grain per gallon'.
Grain -
(gr.) a unit of weight equal to 1/7000th of a pound, or 0.0648 gram.
Grain Per Gallon -
(gpg) a common basis for reporting water analyses in the U.S. and Canadaone grain per U.S. gallons equals 17.12 milligrams per liter or parts per million. one grain per British gallon equals 14.3 milligrams per liter or parts per million.
Gram -
(g) the basic unit of weight (mass) of the metric system, originally intended to be the weight of 1 cubic centimeter of water .
Greensand -
a natural mineral, primarily composed of complex silicates, which possess ion exchange properties.
Hard Water -
water with a total hardness of one grain per gallon or more, as calcium carbonate equivalent.
Hardness -
a characteristic of natural water due to the presence of dissolved calcium and magnesium; water hardness is responsible for most scale formation in pipes and water heaters, and forms insoluble" curd" when it reacts with soaps. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.
Hardness Leakage -
the presence of a consistent concentration of hardness in the effluent from an ion exchange water softener, often due to high concentrations of hardness or sodium in the water being treated.
Head -
a measure of the pressure at a point in a water system, expressed in pounds per square, or in the height of a column of water which could produce the pressure.
Head Loss -
see pressure drop.
Hydraulic -
referring to water or other fluids in motion.
Hydraulic Classification -
a process in which particles pf the same specific gravity may be graded according to size by backwashing or other relative upward flow of water, with the smallest particles tending to rise to the top of the bed, and the largest particles tending to sink to the bottom, because of variations in weight to surface area ratios.
Hydrogen Cycle -
the cation exchange cycle in which the cation exchanger is regenerated with acid, and cations are removed from the solution treated, in exchange for hydrogen ions.
Hydrogen Ion Concentration -
the concentration of hydrogen ions in moles per liter of solution; often expressed as ph.
Hydrologic Cycle -
the water cycle, including precipitation of water from the atmosphere as rain or snow, flow of water over or through the earth, and evaporation or transpiration to water vapor in the atmosphere.
Hydrolysis -
the reaction of a salt with water to form an acid and a base.
Hydroxide -
a chemical compound of an element or elements with the hydroxyl (oh) anion.
Hypochlorite -
the "OCI" anion,; calcium and sodium are commonly used as disinfecting agents
Ion -
an atom, or group of atoms which function as a unit, and has a positive or negative electrical charge, due to the gain or loss of of one or more electrons.
Ion Exchange -
a reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends depends on the affinities of the ion exchanger for the ions present, and the concentrations of the ions in the solution.
Ion Exchanger -
a permanent, insoluble material which contains ions that will exchange reversibly with other ions in a surrounding solution. Both cation and anion exchangers are used in water conditioning.
Ionization -
the process in which atoms gain or lose electrons and thus become ions with positive or negative charges; sometimes used as synonymous with dissociation, the separation of molecules into charged ions in solution.
Ionization Constant -
a constant specific for each partially ionizable chemical compound to express the ratio of the concentration of ions from the compound to the concentration of un-ionized compound.
Iron -
an element often found dissolved in ground water in concentrations usually ranging from zero to 10 ppm. It is objectionable in water supplies because of the staining caused after oxidation and precipitation, because of taste, and because of unsightly colors produced when iron reacts with tannins in beverages such as coffee and tea.
Iron Bacteria -
organisms which are capable of utilizing ferrous iron, either from the water or from steel pipe, in their metabolism, and precipitating ferric hydroxide in their sheaths and gelatinuos deposits. These organisms tend to collect in pipe lines and tanks during periods of low flow, and to break loose in slugs in turbid water to create staining, taste and odor problems.
Jackson Turbidity Unit -
an arbitrary unit of turbidity originally based on a suspension of a specific type of silica with the turbidity measured in a Jackson Candle Turbimeter.
Kilo -
a prefix used to indicate 1000 of the succeeding unit. (abbreviation for kilogram).
Kilograin -
one thousand grains
Kilogram -
one thousand grams
LaneLier's Index -
a calculated number used to predict whether or not a water will precipitate, be in equilibrium with, or dissolved calcium carbonate. It is sometimes erroneously assumed that any water which tend to dissolves calcium carbonate is automatically corrosive.
Lime -
the common name for calcium oxide.
Lime Scale -
hard water scale containing a high percentage of calcium carbonate.
Limestone -
a sedimentary rock, largely calcium carbonate, usually also containing significant amounts of magnesium carbonate.
Liter -
the basic metric unit of volume; 3,785 liters equals 1 U.S. gallon; 1 liter of water weighs 1000 grams.
Maganese -
an element sometimes found dissolved in ground water, usually with dissolved iron but in lower concentrations; causes black stains, and other problems similar to iron.
Magnesium -
one of the elements making up the earths crust, the compounds of which when dis-solved in water make the water hard. The presence of magnesium in water is a factor contributing to the formation of scale and insoluble soap curds.
Manganese Greensand -
greensand which has been processed to incorporate in its pores and on its surface the higher oxides of manganese. The product has a mild oxidizing power, and is often used in the oxidization and precipitation of iron, manganese and/or hydrogen sulfide, and their removal from water.
Media -
the selected materials in a filter that form the barrier to the passage of certain suspended solids or dissolved molecules.
Medium -
singular form of media.
mg/l -
The abbreviation for milligrams per liter.
Micron -
a linear measure equal to one millionth of a meter.
Micron Rating -
the term applied to a filter to indicate the particle size of suspended solids that will be removed. As used in industry standards, this is an "absolute", not "nominal" rating.
Milligram per liter -
a unit concentration of matter used in reporting the results of water andwaste-water analyses. In dilute water solutions, it is practially equal to the part per million, but varies from the ppm in concentrated solutions such as brine. As most analyses are performed on measured volumes of water, the mg/l is a more accurate expression of the concentration, and is the preferred unit of measure.
Millimicron -
a unit of length equal to one thousandth of a micron, often used to express the wave-length of colors of visible light in colormetric analytical procedures. The symbol for the millimicron is "mu".
Mineral -
a term applied to inorganic substances, such as rocks and similar matter found in the earths strata, Minerals normally have definite chemical composition and crystal stucture. The term also applied
to matter derived from minerals, such as inorganic ions found in water.
Mole -
the molecular weight of a chemical compound expressed in grams.
Molecule -
the simplest combination of atoms that will form a specific chemical compound; the smallest particle of a substance which will still retain the essential composition and properties of that substance, and which can be broken down only into atoms and simpler substances.
Most Probable Number -
MPN - the term used to indicate the number or organisms which, according to statistical theory, would be most likely to produce the results observed in certain bacter-iological tests,; usually expressed as a number of 100 ml of water.
Negative Charge -
the electrical charge on an electrode or ion in solution, due to the presence of an excess of electrons.
Neutral -
in electrical systems, the term used to indicate neither an excess or lack of electrons; a condition of balance between positive and negative charges. In chemistry, the term used to indicate a balance between acids and bases; the neutral point on the ph scale is 7.0 indicating the presence of equal numbers of free hydrogen (acidic) and hydroxide (basic) ions.
Noncarbonate Hardness -
water hardness due to the presence of compounds such as calcium and magnesium chlorides, sulfates or nitrates, the excess of total hardness over total alkalinity.
Operating Pressure -
the range of pressure, usually expressed in pounds per square inch, over which a water conditioning device or water system is designed to function.
Osmosis -
a process of diffusion of a solvent such as water through a semipermeable membrane which will transmit the solvent but impede most dissolved substances. The normal flow of solvent is from the dilute solution to the concentration solution.
Oxidation -
a chemical process in which electrons are removed from an atom, ion or compound. The addition of oxygen is a specific form of oxidation, while the rusting of iron is a slow form.
Particle Size -
as used in industry standards, the size of a particle suspended in water as determined by its smallest dimension, usually expressed in microns.
Parts Per Million -
(ppm) a common basis for reporting the results of water and wastewater analyses, indicating the number of parts by weight of a dissolved or suspended constituent, per million parts by weight of water or other solvent. In dilute water solutions, one part per million is practically equal to one milligram per liter, which is the preferred unit.
Pathogen -
an organism which may cause disease.
Permanent Hardness -
water hardness due to the presence of the chlorides and sulfates of calcium and magnesium, which will not be precipitated by boiling. this term is largely replaced by "noncarbonate hardness".
PH -
the reciprocal of the logarithm of the hydrogen ion concentration. The ph scale is from zero to 14,and 7.0 is the neutral point, indicating the presence of equal concentrations of free hydrogen and hydroxide ions, ph values below 7.0 indicate increasing acidity, and ph values above 7.0 indicate increasing base concentrations.
Portable Exchange -
a term applied to water softeners and filters which are designed for connection to a water system with special fittings and disconnection and transport to a central station or plant for regeneration or servicing.
Positive Charge -
the electrical charge of an electrode or ion in solution due to the removal of electrons.
PPM -
the abbreviation for part per million.
Precipitate -
to cause a dissolved substance to form a solid particle which can be removed by settling or filtering, such as the removal of dissolved iron by oxidation, precipitation, and filtration. The term is also used to refer to the solid formed, and to the condensation of water in the atmosphere to form rain or snow.
Pressure Differential -
the difference in pressure between two points in a system due to the differences in elevation and/or pressure drop due to flow.
Pressure Drop -
a decrease in water pressure during flow due to internal friction between molecules of water, and external friction due to irregularities or toughness in surfaces past which the water flows.
Rated Capacity -
the basis for calculating the period of time, or number of gallons delivered by a water softener or filter, between regenerations or servicing, as determined under specific test conditions.
Rated Pressure Drop -
the pressure drop of water softener or filter at the rated service flow, with clean water at a temperature of 60degrees. with a freshly regenerated and/or backwashed softener or filter, as determined under standard test conditions.
Rated Service Flow -
the manufacturer's specified maximum flow rate at which a water softener will deliver soft water, or a filter will deliver quality water as specified for its type, as determined under standard test conditions. A manufacturer may also specify a minimum flow rate or a range of service flows.
Rated Softener Capacity -
a water softener capacity rating based on grains of hardness re-moved while producing soft water between successive regenerations, as determined under standard test conditions.
Raw Water -
untreated water, or any water before it reaches a specific water treatment device or process.
Red Water -
water which has a reddish or brownish appearance due to the presence of precipitated iron and/or iron bacteria.
Reduction -
a chemical process in which electrons are added to an atom, ion or compound.
Regenerant -
a solution of a chemical compound used to restore the capacity of an ion exchange system. Sodium chloride brine is used as a regenerant for ion exchange water softeners, and acids and bases are used as regenerants for the cation and anion resins used in demineralization.
Regeneration -
in general, includes the backwash, brine, and fresh water rinse steps, necessary to prepare a water softener exchange bed for service after exhaustion. Specifically, the term may be applied to the 'brine' step in which the sodium chloride solution is passed through the exchanger bed. The term may also be used for similar operations relating to the demineralizers and certain filters.
Regeneration Level -
the quantity of regenerant used in the regeneration of an ion exchange unit , usually expressed in pounds per regeneration and/or pounds per regeneration per cubic foot of ion exchange.
Residual -
the amount of a specific material remaining in the water following a water treatment process. May refer to material remaining as a result of incomplete removal or to material meant to remain in the treated water.
Resin -
synthetic organic ion exchange material, such as the high capacity exchange resin widely used in water softeners.
Reverse Deionization -
the use of the anion exchange resin ahead of the cation exchange resin in a deionization system.
Reverse Osmosis -
a process for the removal of dissolved ions from water, in which pressure is used to force the water through semipermeable membrane, which will transmit the water but reject most other dissolved materials.
Saline Water -
water containing an excessive amount of dissolved salts, usually over 10,000 mg/l.
Salt -
the common name for the specific chemical sodium chloride, used in regeneration of ion exchange water softeners.
Sequester -
a chemical reaction in which certain ions are bound into a stable, water soluble compound, thus preventing undesirable action by the ions.
Sequestering Agent -
a chemical sometimes fed into water to tie up undesirable ions, and eliminate the normal effects of the ions.
Soap -
one of a class of compounds which possesses cleaning properties, Sodium and potassium are soluble and useful.
Soda ash -
the common name for sodium carbonate, chemical compound used as an alkaline builder in some soap and detergent formulations, to neutralize acid water, and in the lime-soda ash water treatment process.
Sodium -
an ion found in natural water supplies, and introduced to water in the ion exchange water softening process.
Sodium Chloride -
the chemical name for common salt, used in regeneration.
Soft Water -
any water which contains less than 1.0 gpg(17.1 mg/l) of hardness minerals, expressed as calcium carbonate.
Softened Water -
any water that is treated to reduce hardness minerals to 1.0 gpg or less.
Solute -
the substance which is dissolved in a solvent. Dissolved solids, such as the minerals found in water, are solutes.
Solvent -
the liguid, such as water, in which other materials (solutes) are dissolved
Specific Gravity -
the ratio of the weight of a specific volume of a substance compared to the weight of the same volume of pure water at 4 degrees c.
Sulfate Reducing Bacteria -
a group of bacteria which are capable of reducing sulfates in water to hydrogen sulfide gas, thus producing obnoxious tastes and odors. These bacteria have no sanitary significance, and are classed as nuisance organisms.
Sulfur -
a yellowish solid element. The term is also used as a slang expression to refer to water containing hydrogen sulfide gas.
TDS -
the abbreviation for "total dissolved solids"
Threshhold -
a very low concentration of a substance in water. The term is sometimes used to indicate the concentration which can just be detected.
Titration -
an analytical process in which a standard solution in a calibrated vessel is added to a measured volume of sample until an end point, such as color change, is reached. From the volume of the sample and the volume of standard solution used, the concentration of a specific material may be calculated.
Total Acidity -
the total of all forms of acidity, including mineral acidity, carbon dioxide and acid salts. Total acidity is usually determined by titration with a standard base solution to the phenolphathlein endpoint.
Total Alkalinity -
the alkalinity of a water as determined by titration with standard with standard acid solution to the methyl orange endpoint(ph approx. 4.5); sometimes abbreviated as "M alkalinity" Total alkalinity includes many components, such as hydroxides, carbonates, and bicarbonates.
Total Dissolved Solids -
the weight of solids per unit volume of water which are in true solution, usually determined by the evaporation of a measured volume of filtered water, and of the residue weight.
Total Hardness -
the sum of all hardness constituents in a water, expressed as their equivalent concentration of calcium carbonate. Primarily due to the calcium and magnesium in solution, but may include small amounts of metals such as iron which can act like calcium and magnesium in certain reactions.
Total Solids -
the weight of all solids, dissolved and suspended organic and inorganic, per unit volume of water; usually determined by the evaporation of a measured volume of water at 105c in a preweighed dish.
Turbidity -
a measure of the amount of finely divided suspended matter in water, which causes the scattering and absorption of light rays.
