This study concerns the diffraction problem relevant to a plane wave normally incident with respect to a 90° wedge. A penetrable material with finite conductivity forms the structure. A high-frequency solution is here obtained by adopting a physical optics approximation for the equivalent electric and magnetic surface currents involved in the radiation integrals used to represent the fields scattered in the inner region of the wedge and the surrounding space. Uniform asymptotic evaluations of such integrals lead to closed form expressions for the diffraction coefficient in terms of the transition function of the uniform theory of diffraction and the Fresnel coefficients for the reflection and transmission mechanisms. No limitation exists on the loss tangent of the medium. Comparisons with numerical tools assess the effectiveness of the proposed solutions for the field diffracted in the inner and outer regions.