The area near the electrode surface is called the diffusion layer, and to understand electrochemistry, it is crucial that students have a knowledge of the phenomena occurring at the diffusion layer. Here, we present a demonstration and activity to visualize and analyze the expansion of a micrometer-sized diffusion layer. The electrode process involves distinct color changes of acid--base indicators in response to electrochemically generated hydroxide ion (OH-), from water electrolysis in a homemade thin-layer electrochemical cell. A kid's optical microscope equipped with a digital eyepiece camera was used to observe and record the formation and expansion of the diffusion layer. Analyzing the time-dependent changes in the colorful diffusion layer enables students to derive the diffusion coefficient (D) of electrochemically generated OH-. The imaging tool presented in this activity aids in the explicit visualization and interpretation of electrode reactions and provides an excellent opportunity to discuss concentration profiles near the electrode, diffusion layer, and diffusion coefficient.