Using Mie scattering to determine the wavelength dependant refractive index of polystyrene beads with changing temperature

Polystyrene beads are used as test aerosol. A contact-less technique is reported for determining the size and refractive index of a solid aerosol particle as a function of wavelength and temperature (20-234°C). Polystyrene beads of 2μm diameter were optically trapped in air, in the central orifice of a ceramic heating element and Mie spectroscopy (0.480-0.650μm) was then used to determine the radius and refractive index (to precisions of 0.8nm and 0.0014) of 8 beads as a function of heating and cooling. Refractive index, n, as a function of wavelength, λ, in microns, and temperature, T, in centigrade, was found to be n = 1.5753 - (1.7336 × 10¹⁴) T + (9.733 × 10^-3) λ^-2 in the temperature range 20 < T < 100 °C and n = 1.5877 - (2.9739 × 10^-4) T + (9.733 × 10^-3) λ^-2 in the temperature range 100< T <234 °C, significantly extending the temperature and wavelength range for refractive index of polystyrene. The work represents a step change in measuring the refractive index of materials in an absolute manner with high precision as a function of temperature and wavelength simultaneously. The work also demonstrates a new development in measurement of refractive index as a function of wavelength and temperature for measuring phase changes.