The transparency or opacity of a paint layer depends on the amount of scattering and absorption of the light. Scattering in paint films is the deflection of light rays by pigment particles suspended in a binding medium. Scattering depends on the difference in index of refraction between the paint pigment particle and the surrounding binding medium. If the difference in the refractive indices of the pigment particle and the binding medium is large, then the pigment deflects light effectively. For example, a particle of titanium oxide white pigment with a refractive index of 2.5 suspended in a medium of linseed oil with a refractive index of 1.5 will scatter light very effectively. A particle of ultramarine blue pigment with refractive index of 1.5 suspended in the same linseed oil medium will appear transparent because the refractive indices are closer on both sides of the interface, and little scattering will take place. Painters utilize the differences in opacity and transparency resulting from these relationships in organizing many aspects of their paintings, including the mixing of colors and the sensation of luminosity in the paint films.
The size of the pigment particle also influences the amount of scattering. Particles of about the same size as the wavelengths of visible light are more effective in scattering visible light than particles much larger or smaller. For example, centimeter size titanium oxide crystals are transparent, but when ground down to about 500 nanometers they scatter visible light very strongly. The amount of scattering also depends on the density of particles (number per unit volume) in the medium. The higher the density, the greater the amount of scattering. White correction fluid is a dense aggregation of titanium oxide particles suspended in a liquid medium. The opacity of the correction fluid is determined by the density as well as the difference in refractive index and the particle size.
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