Optical properties of N-isopropylacrylamide (NIPA) microgel spheres in water have been investigated using light-scattering and turbidity methods. Two batches of NIPA microgel spheres with hydrodynamic radii of 132 and 216 nm were synthesized in water at 25 degreesC. Concentrations ranging from similar to0.01 to similar to14 wt %, were obtained by dilution-condensation of the dispersions. The hydrodynamic radius distribution, the radius of gyration, and the molar mass of the microgel spheres were determined by combining dynamic and static light scattering. As the polymer concentration increases, the microgel spheres in dispersions exist in the liquid, crystalline, and glass states, while the optical appearance of the dispersions changes progressively from transparent to cloudy to colored (pink, green, blue, and purple gradually) and to transparent. The formation of large colloidal crystals in a very narrow concentration range (ca. 3similar to5 wt %) at room temperature (similar to18-22 degreesC) yields iridescent patterns from typical Bragg diffraction. For a colored dispersion, the turbidity exhibits a sharp shoulder-shaped increase at a certain wavelength), that decreases linearly with decreasing interparticle distance. It is found that for a dilute dispersion light scattering is the main cause of the turbidity, while for a concentrated dispersion in which microgel spheres are close-packed, interparticle interference becomes dominant. Heating a concentrated NIPA microgel dispersion from room temperature to 35 degreesC leads to a continuous increase of turbidity. The ordered structure of microgel spheres was melted and transformed into a disordered liquid state at about 30 degreesC, which was lower than the volume phase-transition temperature of similar to34 degreesC. Phase inversion can be obtained either by condensation of the sample at room temperature or by heating a concentrated sample.