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# What is the Meaning of Refractive Index and Absorptivity?

Refractive index is the ratio of light speed in vacuum and light speed in other materials. The higher the refractive index of the sample, the greater the refractive ability of incident light.

Absorptivity is the ratio of the amount of heat radiation absorbed by light on a particle to the total heat radiation energy projected on the particle. In the software of laser particle size analyzer, refractive index is expressed by a complex number where the real part is the actual refractive index and the imaginary part is absorptivity.

The refractive index and absorptivity of common compounds can be found in the software of particle size analyzer and powder handbook. For special samples, the refractive index can be measured directly. Besides, the refractive index of one known sample may be used to replace refractive index of another sample if they have similar optical properties. For example, the refractive index of 'Fe' could be used to replace the refractive index of alloys.

The refractive index of unknown samples may also be estimated based on the color of the samples. For example, the refractive index of black samples is 1+, that of dark gray samples is 1-0.1, and that of transparent to white samples is 0-0.01.

The suitability of selected real and imaginary parts in refractive index can also be verified by the 'fitting precision' in the test results. The lower the fitting precision, the more accurate the refractive index. In general, it indicates the refractive index is accurate when the fitting precision <0.5. A large error of refractive index occurs when the fitting precision >1. The real and imaginary parts of the refractive index can be modified accordingly until the "fitting precision" meets the requirements.