Metals like Li, Na, K and Cs are photosensitive even to visible light because they

1. Alkali metals have loosely held valence electrons in the outermost shell. 2. Their work functions are low, of order 1-3 eV. 3. Visible light (1.7-3.1 eV) exceeds these work functions for many alkali metals. 4. So visible light can eject photoelectrons from Li, Na, K, Rb, Cs. _Source: syllabus Physics, Ch 11 "Dual Nature of Radiation and Matter", §11.3.2_