ABSTRACT
Light is electromagnetic radiation in the wavelength range from 3 nm to 30 μm. Visible light is the part of electromagnetic radiation to which the human eye is sensitive, and the wavelength of it ranges from 400 nm to 750 nm. When a light beam illuminates a particle having a dielectric constant different from unity, a part of the light will be reflected on the surface of particle, and the rest of the light will pass into the particle, as shown in Figure 2.1. The part of the light passed into the particle will go out as refraction, and the rest will be absorbed. When the particle size is much larger than the wavelength of the incident light, the light beam is diffracted near the particle. The scattering phenomenon of light includes diffraction, deflection, and refraction of light. The net results of the absorption and scattering caused by the particle are known as the extinction of incident light. In describing such optical phenomena, the size parameter α defined by the following expression may be important:
α5 lDp (2.1)
where Dp is the particle diameter and l is the wavelength of the incident light. Another important definition is the refractive index. The refractive index of the medium is given
by the ratio of the velocity of light in a vacuum, c, to that in the medium, v:
m c v0 5 (2.2)
This value is always larger than unity.
