From the experimental study of photo-electric effect, it is seen that photoemission occurs only when the light has a frequency greater than a certain value. Photocurrent is zero when the frequency of light is below this value. This limiting value of frequency is called the threshold frequency. The corresponding wavelength is called the threshold wavelength. Different metals are found to have different threshold frequencies.

Photoemission does not take place when the incident frequency is below the threshold value, however intense the light may be. On the other hand, photo-emission readily takes place when the frequency is above the threshold, however feeble the light may be.

Thus, it appears that the photoelectric effect is a frequency dependent phenomenon and not an intensity dependent phenomenon.

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By :-   Dr. A. W. Pangantiwar