Two-dimensional materials hold a great promise for developing extremely fast, compact and inexpensive optoelectronic devices. A molybdenum disulphide (MoS2) monolayer is an important example which shows strong, stable and gate tunable optical response even at room temperature near excitonic transitions. However, optical properties of a MoS2monolayer are not documented well. Here we investigate the electric field effect on optical properties of a MoS2 monolayer and extract the dependence of MoS2 optical constants on gating voltage. The field effect is utilised to achieve ~10% visible light modulation for a hybrid electro-optical waveguide modulator based on MoS2. A suggested hybrid nanostructure consists of a CMOS compatible Si3N4 dielectric waveguide sandwiched between a thin gold film and a MoS2 monolayer which enables a selective enhancement of polarised electro-absorption in a narrow window of angles of incidence and a narrow wavelength range near MoS2 exciton binding energies. The possibility to modulate visible light with 2D materials and the robust nature of light modulation by MoS2 could be useful for creation of reliable ultra-compact electro-optical hybrid visible-light modulators.