Abstract: Methods and apparatus for irradiating biological tissue by EM radiation having radiation frequency(ies) of at most 10 Gigahertz are disclosed herein. In some embodiments, the tissue is irradiated by passing converging EM waves (e.g. generated using an ellipsoidal mirror 110) through a surrogate medium having a specially shaped ENTRY_SURFACE via which the converging EM waves enter the surrogate medium. In some embodiments, a refractive index at a sub-10 Gigahertz of the surrogate medium is at least 2 or at least 3 or at least 5 and/or substantially matches a refractive index of an irradiated biological tissue. In some embodiments, converging EM waves are formed within the surrogate medium. Some embodiments relate to methods and apparatus for irradiating neuron(s), for example, to non-invasively stimulating or otherwise modify a behavior of neuron(s) using focused or non-focused EM radiation.