The same photosensitive substance
gives different response to light of different wavelengths For example,
ultraviolet light gives rise to  photoelectric effect in copper  while green or
red light does not Note that in all the above experiments, it is found that, if frequency of
the incident radiation exceeds the threshold frequency, the photoelectric
emission starts instantaneously without any apparent time lag, even if
the incident radiation is very dim It is now known that emission starts in
a time of the order of 10– 9 s or less