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Influence of Particle Size on Optical Characteristics in Transparent and Opaque Materials

Transparency is defined in terms of a substance’s ability to transmit light. If an object viewed through it can be clearly recognised, the substance is transparent. Clear window glass is an obvious example of a transparent solid material, while many liquids and most gases are also transparent.

Particles which absorb light can affect the degree of transparency for particular wavelengths of light, causing the transparent material to become tinted. For instance, certain metal oxides in glass, or coloured smoke in the air, can have this effect. As the concentration of these particles increases, transparency decreases.

Some materials, such as frosted glass, will transmit light, but not so well as to allow clear identification of objects viewed through them. These substances may be referred to as translucent rather than transparent. Materials such as metal and wood, which do not transmit light at all, are referred to as opaque. Opacity, which is the reciprocal of translucency, is the ability of a material to block the transmission of light.

A further optical property of some substances is opalescence. This occurs when particles scatter or otherwise interfere with the light to produce colour effects. Particles smaller than the wavelength of the light can cause Rayleigh scattering, with an intensity that increases substantially as wavelength decreases. The fact that blue light is scattered more intensely than red light accounts for the sky’s blue colour. Particles larger than the light’s wavelength can generate Mie scattering, whose intensity is less dependent on wavelength. Comparison of different materials shows a gradation from opalescence to opacity.

A useful tool for investigating these properties is the ANALYSETTE 22 laser particle sizer from FRITSCH. Using this instrument, researchers have looked at, for example, the particle size distribution of cryolite, which is used in frosted glass. It is clear from such work that different particle sizes produce different optical effects.

A background measurement is taken and then a saturated cryolite sample is added. This avoids destabilisation in solution, which would cause shifting of the measuring curve towards smaller particle sizes with each successive measurement. Measuring the sample in alcohol, using a small volume dispersion unit, is another possible approach.

Date added: 2013-04-11 23:26:45