Routinely used in quantitative determination of various analytes, UV-vis spectroscopy is commonly taught in undergraduate chemistry laboratory courses. Because the technique measures the absorbance of light through the samples, losses from reflection and scattering by large molecules interfere with the measurement. To emphasize the importance of light scattering in UV-vis spectroscopy, a laboratory experiment is described that guides students to study the hydrolysis of starch by amylase as a function of temperature. The reducing sugar produced from the hydrolysis is determined by dinitrosalicylic colorimetric assay via two methods: UV-vis spectroscopy and a smartphone-based colorimetry, which is capable of quantifying the concentration of a chromogenic compound in an opaque sample. From this experiment, students can directly learn to measure the activity of amylase, use a smartphone to perform colorimetry, observe the effect of light scattering, and gain a better understanding of UV-vis spectroscopy.