Hallwachs and  Lenard also observed that when
ultraviolet light fell on the emitter plate, no electrons were emitted at all
when the frequency of the incident light was smaller than a certain
minimum value, called the threshold frequency This minimum frequency
depends on the nature of the material of the emitter plate It was found that certain metals like zinc, cadmium, magnesium, etc ,
responded only to ultraviolet light, having short wavelength, to cause
electron emission from the surface