Abstract: The optoelectronic interface and refractive index display device has a light-emitting diode the light from which is fed through an optical waveguide to a sensing element. A planoconvex lens collimates the light which then passes through a regular cylindrical element to a circular cone. Total reflection occurs at the laterial conical surface thereof when the circular cone is in a gaseous atmosphere. The reflected light is fed through the regular cylindrical element, the planoconvex lens, and then through a common waveguide and receiving waveguide to a photodiode. When the circular cone is immersed wholly or partially in liquid, the light coupled into the sensing element experiences no significant total reflection. The signal generated by photodiode decreases accordingly. The planoconvex lens reduces signal loss.