Interference fringes produced by an optical wedge system filled with aqueous electrolyte have been used to study the changes of optical density in an electrolyte when current is flowing. Whilst the relationship between optical density and concentration has not been completely evaluated for such systems, yet it may be said that in terms of optical density the anode and cathode layers at truly working electrodes show three components, two of which vanish rapidly when the current is switched off whilst the third, which fades much less rapidly may be identified with the concentration polarization layer studied by Brenner.