Question

Red light, however bright it is, cannot produce the emission of electrons from a clear zinc surface. But even weak ultraviolet radiation can do so. Why?

Answer 1

The frequency of red light is less than the threshold frequency of zinc surface. Hence, it cannot cause photoelectric emission from zinc surface, whatever may be its intensity. The frequency of ultraviolet radiation is greater than the threshold frequency of zinc surface. Hence, even weak ultraviolet radiation can cause photoelectric emission.