Figure 1.

Illustration of some optical phenomena to be exploited for the identification of microplastic (MP) properties. (a) Overview of a standard experiment: the incident light is reflected, transmitted, and absorbed. (b) Interference: If the MP has two smooth surfaces, multiple reflections inside the film lead to an interferogram. (c) Speckle: Rough surfaces will generate a speckle originating from multiple interferences. (d) Diffraction: edges of the MP particles can diffract light beams to create organized patterns. (e) Absorption: the output amplitude is lower than the incident one. (f) Fluorescence: absorbed light energy is re-emitted at other wavelengths when molecules relax. (g) Raman scattering: specific light frequencies excite vibrational states of molecules, which lead to the emission of secondary photons at a slightly shifted frequency (Stokes and anti-Stokes).