question: Why does a fabric appear darker when wet ?
comment: A perception of 'darkness' means that not much light is reflected from the surface to your eye. A film of water makes some of the light refract/scatter, preventing it from reaching your eyes, thus creating a 'darker' image. ...at least that is the explanation for bodies of water. I am not 100% sure whether that is also the explanation for fabric.
passages: Sheer fabric
  
  Sheer fabric is fabric which is made using thin thread and/or low density of knit and which results in a semi-transparent and flimsy cloth. Some fabrics become transparent when wet. 
  
  Section::::Overview.
   The fabric passes over brushes to raise the fibers, then passes over a plate heated by gas flames. When done to fabrics containing cotton, this results in increased wetability, better dyeing characteristics, improved reflection, no "frosty" appearance, a smoother surface, better clarity in printing, improved visibility of the fabric structure, less pilling and decreased contamination through removal of fluff and lint.
   Latex rubber, which is naturally translucent, or plastics can be made into clothing material of any level of transparency. Clear plastic is typically only found in over-garments, such as raincoats. The use of translucent latex rubber for clothing can also be found in fetish clothing. Some materials become transparent when wet or when extreme light is shone on it, such as by a flashbulb.
  
  Section::::18th and 19th centuries.
   In Bangladesh, textile manufacturing is a major industry. In this industry, wet processing plays a vital role in the area of pre-treatment, dyeing, printing and finishing of both fabrics and apparels. But coloration in fiber stage or yarn stage is also included in the wet processing division.
  
  All the processes of this stream are carried out in an aqueous state or aqueous medium. The main processes of this section include:
  
  BULLET::::- Singeing
  
  BULLET::::- Desizing
  
  BULLET::::- Scouring
  
  BULLET::::- Bleaching
  
  BULLET::::- Mercerizing
  
  BULLET::::- Dyeing
  
  BULLET::::- Printing
  
  BULLET::::- Finishing
  
  Section::::Pre-treatment.:Singeing.
   Section::::Color vision.
  
  Water attenuates light due to absorption which varies as a function of frequency. In other words, as light passes through a greater distance of water color is selectively absorbed by the water. Color absorption is also affected by turbidity of the water and dissolved material.
   The "pump" layer and the shell fabric both rely on a durable water repellent treatment; this treatment in combination with the fabric structure of the "pump" layer creates the capillary depression effect that allows water to be driven away from the wearer. In common with other waterproof garment technologies, this water repellent treatment must be maintained in order to remain effective.
  
  Section::::Characteristics.
   For [[Desmond Morris]], water on the skin is seen as mimicking the sweat of sexual arousal.
  
  Section::::Fashion and pop.
  
  The [[Sixties]] success of the [[Merseybeat]] saw wetlook pvc coats coming down from Liverpool to enter the London fashion scene. A few decades later, a pop [[Ladette]] might feel ambivalent to find herself posing in a pvc catsuit.
  
  Section::::Classical prototypes.
  
  BULLET::::- [[New Kingdom of Egypt]] poetry has a girl telling her lover: “It is pleasant to go to the pool...That I may let you see my beauty in my tunic of finest royal linen When it is wet”.
   In beam dyeing a perforated warp beam is used instead of the spools used in package dyeing.
  
  Space dyeing is used to produce yarns with multiple colors.
  
  In general, yarn dyeing provides adequate color absorption and penetration for most materials. Thick and highly twisted yarns may not have good dye penetration. This process is typically used when different colored yarns are used in the construction of fabrics (e.g. plaids, checks, iridescent fabrics).
  
  Section::::Dyeing.:Fabric dyeing.
   The purposes of calendering are to upgrade the fabric hand and to impart a smooth, silky touch to the fabric, to compress the fabric and reduce its thickness, to improve the opacity of the fabric, to reduce the air permeability of the fabric by changing its porosity, to impart different degree of luster of the fabric, and to reduce the yarn slippage.
  
  Section::::Finishing.:Raising.
   As opposite to air, water attenuates light exponentially. This results in hazy images with very low contrast. The main reasons for light attenuation are light absorption (where energy is removed from the light) and light scattering, by which the direction of light is changed. Light scattering can further be divided into forward scattering, which results in an increased blurriness and backward scattering that limits the contrast and is responsible for the characteristic veil of underwater images. Both scattering and attenuation are heavily influenced by the amount of organic matter dissolved in the water.
   Linen fabrics have a high natural luster; their natural color ranges between shades of ivory, ecru, tan, or grey. Pure white linen is created by heavy bleaching. Linen fabric typically varies somewhat in thickness and is crisp and textured, but it can in some cases feel stiff and rough, and in other cases feel soft and smooth. When properly prepared, linen fabric has the ability to absorb and lose water rapidly. Linen can absorb a fair amount of moisture without feeling unpleasantly damp to the skin, unlike cotton.
   These fiber optic coating layers are applied during the fiber draw, at speeds approaching . Fiber optic coatings are applied using one of two methods: "wet-on-dry" and "wet-on-wet". In wet-on-dry, the fiber passes through a primary coating application, which is then UV cured—then through the secondary coating application, which is subsequently cured. In wet-on-wet, the fiber passes through both the primary and secondary coating applications, then goes to UV curing.
   Factors that affect the level of sun protection provided by a fabric, in approximate order of importance, include weave, color, weight, stretch, and wetness. The less open or more dense the fabric (weave, weight, stretch), the better the protection. Getting a fabric wet reduces the protection as much as half, except for silk and viscose which can get more protective when wet. Polyester contains a benzene ring that absorbs UV light. In addition, UV absorbers may be added at various points in the manufacturing process to enhance protection levels. In 2003, chemical company BASF embedded nanoparticles of titanium dioxide into a nylon fabric, which can be used for sun protective clothing that maintains its UV protection when wet.
   Sometime after nylon-backed neoprene appeared, the blind stitch method was developed. A blindstitch sewing machine uses a curved needle, which does not go all the way through the neoprene but just shallowly dips in behind the fabric backing, crosses the glue line, and emerges from the surface on the same side of the neoprene. This is similar to the overlock stitching used for teeshirts and other garments made from knitted fabrics.
   BULLET::::- Troy took the galoshes of a Cornell lecturer and painted human feet on them. Then he covered his handiwork with a black paint that would wash off. When the lecturer used the galoshes in the rain, he looked like he was walking with bare feet.
   The process of singeing is carried out for the purpose of removing the loose hairy fibers protruding from the surface of the cloth, thereby giving it a smooth, even and clean looking face. Singeing is an essential process for the goods or textile material which will be subjected to mercerizing, dyeing and printing to obtain best results from these processes.
   In the textile industry, loose fibres protruding on the surface of textile goods are singed to remove them. When done to fabrics containing cotton, this results in increased wettability, better dyeing characteristics, improved reflection, no "frosty" appearance, a smoother surface, better clarity in printing, improved visibility of the fabric structure, less pilling and decreased contamination through removal of fluff and lint.
   An example of readily tunable wettability is found with special developed fabrics. By stretching a dip-coated commercial fabric, contact angles were typically allowed to increase. This is largely caused by an increase in tower spacing. However, this trend does not continue towards greater hydrophobicity with higher strain. Eventually, the Cassie-Baxter state reaches an instability and transitions to the Wenzel state, soaking the fabric.
   Section::::Directional fabrics.
   A plant fibre (or other hydrophilic material) that has been exposed to the atmosphere will usually contain some water even if it feels dry. The water can be driven off by heating in an oven, leading to a measurable decrease in weight, which will gradually be regained if the fibre is returned to a 'normal' atmosphere. This effect is important in the textile industry, where the proportion of a material's weight made up by water is called the "moisture regain". 
   Section::::Process.
   Wet processing is usually done on the manufactured assembly of interlacing fibers, filaments, and/or yarns having substantial surface (planar) area in relation to its thickness, and adequate mechanical strength to give it a cohesive structure. In other words, wet process is done on manufactured fabric. The processes of this stream is involved or carried out in aqueous stage and thus it is called wet process which usually covers pre-treatment, dyeing, printing and finishing.
   Hydroentanglement is sometimes known as spunlacing, this term arising because the early nonwovens were entangled on conveyors with a patterned weave which gave the nonwovens a lacy appearance. It can also be regarded as a two-dimensional equivalent of spinning fibres into yarns prior to weaving. The water pressure has a direct bearing on the strength of the web, and very high pressures not only entangle but can also split fibres into micro- and nano-fibres which give the resulting hydroentangled nonwoven a leatherlike or even silky texture. This type of nonwoven can be as strong and tough as woven fabrics made from the same fibres.
   It is also important to note that each fluid has its own minimum flow rate. The minimum flow rate is the smallest amount of dye at a given moment to keep the curtain flowing continuously. The minimum flow rate is directly proportional to the surface tension while viscosity is inversely proportional.
   An important and oldest textile finishing is brushing or raising. Using this process a wide variety of fabrics including blankets, flannelettes and industrial fabrics can be produced. The process of raising consists of lifting from the body of the fabric a layer of fibers which stands out from the surface which is termed as "pile". The formation of pile on a fabric results in a "lofty" handle and may also subdue the weave or pattern and color of the cloth.
  