|MadSci Network: Chemistry|
Brenda, Believe it or not, white, gray and black are the same "color". They share a common property: They reflect or transmit all the wavelengths of visible light equally or at least with the same response as your eyes. Notice I said your eyes not our eyes. Hoping that you are wondering a bit I will give you a couple of references that were just mentioned elsewhere in the Mad Scientist, two books by Marcel Minnaert, "Light and Color in the Outdoors” and the classic "The Nature of Light and Colour in the Open Air". Both are available from Amazon.com and are probably in your local library. These books are marvelous! After reading them you will look at everything from a fantastically new viewpoint. Also the Kodak "Here’s How" series on [mostly Color] Photography gives simple, precise, and interesting descriptions of color, pigments, vision and photography. A white material or pigment both reflects and/or transmits almost all, usually more than 90% or so, of all the wavelengths of light falling on it. A material such as a polished metal, which reflects almost all of the light, is not white because it doesn't transmit any light. The black areas in a black and white photo are usually [they can also be platinum or palladium or other metals] very small silver particles. These reflect but do not transmit light so the very fine particles end up absorbing light and they look gray or black depending on the depth of the particles. A white pigment in the bulk such as a large crystal or a sheet of clear, colorless, glass or plastic reflects a small amount of the incident light and transmits the remainder. If the material is ground into fine particles there are more reflecting surfaces and more light is reflected backwards by a process known as diffuse reflection. The material becomes "white". As the particles become smaller and approach molecular size they lose the ability to reflect and again become colorless. Two examples: An ice cube, if properly frozen is clear; snow, which is small crystals, is white; molten snow which consists of water molecules randomly arranged is again clear [and almost colorless. pure water does have a blue color]. A large salt or sugar [rock candy] crystal is clear and colorless. The sugar and salt you buy are small clear crystals but look white in bulk. Powdered sugar doesn't look clear at all. When you dissolve sugar, giving dissolved sugar molecules, or salt, giving dissolved sodium and chloride ions, in water; the solutions are again clear and colorless. A white pigment also has some other important physical and chemical properties. It must be chemically inert, usually insoluble in water and capable of being ground to a fine powder or suspended as small droplets. It must also be obtainable in a pure chemical state so it doesn't fluoresce or phosphoresce, that is give off light different from the light shining on it. There is another useful property. It should have a high refractive index, a high ability to bend light that strikes it obliquely. Some materials that are used as or act as white pigments: Paper, flour, powdered sugar, whipped cream, whipped egg white [meringue, mayonnaise], whipped fat [Crisco], water [clouds, fogs]. These are useful but do lack the property of chemical stability or insolubility so they are usually used only in food or simple play crafts. There are many white materials that are more chemically inert and are used as pigments. Common ones are silica[SiO2], chalk[calcium carbonate[CaCO3], Titania[titanium dioxide, TiO2], Zirconia[Zirconium dioxide, ZrO2], baryta[barium sulfate,BaSO4], gypsum[calcium sulfate, CaSO4], powdered glass, zinc oxide[ZnO] and zinc sulfide[ZnS]. Probably the most common in modern paints are silica and titanium dioxide the first because it is inexpensive the second because it has a high refractive index and makes paint more opaque. Both are very chemically inert. Pigments that are not used any more because of their toxicity are lead carbonate[PbCO3, white lead] and lead oxide[litharge, PbO]. These have high opacity or covering power because of their high refractive index. A pigment works by the process of diffuse reflectance. The finely divided pigment reflects incident light back towards the source. This means that if one looks at the transmitted light from the other side it looks gray. The best examples of this are clouds; when the sun shines on them they are brilliant white; when they get between you and the sun they can be a dismal gray. Photographic paper, which is designed to reflect a lot of light, can also look gray by transmitted light. . There are also physical, physiological and psychological aspects to color. The physical: A white material reflects almost all of each the light wavelengths incident on it. This is not restricted to white light. A striking example is working under an orange photographic safe light. I once brought a small beaker of potassium dichromate [a bright orange material] into a dark room and placed it on the table and went about setting up and of course letting my eyes and brain become accustomed to the light. When I went to look for the beaker I couldn't find it. All there was, was a beaker containing a bright WHITE material. I finally had to turn the room lights on to check and the chemical turned orange right before my eyes. A material that reflects all of the light incident on it looks white even if it isn't white under other lighting conditions. A blue object in blue light looks white; a red object in red light looks white. White is dependent on the lighting conditions, tungsten light, sunlight, fluorescent light, each gives its own character to white. Each person’s eye[s] and most photocell types respond differently to light wavelengths so they have different whites. The eye brain combination adapts and after a time makes the overall color distribution look white. Photographic film doesn't do this so different lighting will give different colorcasts to a photo. Finally there is a strong mental aspect. Some things you simply expect to look white. Your brain either makes them white or you can become confused when they don't look white. For example, skin tones under mercury vapor lighting will look garish and even frightening. Color, even white, is a fascinating subject. Go out and look for the differences in light and in the colors of things.
Try the links in the MadSci Library for more information on Chemistry.