Threshold frequency and no time lag

Now keep the intensity the same and change the frequency. The saturation current comes out the same for every frequency, but the stopping potential changes: it is more negative for higher frequencies, V₀₃ > V₀₂ > V₀₁ when ν₃ > ν₂ > ν₁ (Fig. 11.4).

Higher-frequency light therefore gives photoelectrons a larger maximum kinetic energy, and a larger retarding potential is needed to stop them.

Plotting stopping potential against frequency gives a straight line for each metal (Fig. 11.5). V₀ varies linearly with ν, and the line meets the frequency axis at a cut-off frequency ν₀ where the stopping potential is zero.

Two conclusions follow. Kmax rises linearly with frequency but does not depend on intensity. And below ν₀ there is no emission at all, however intense the light. This cut-off ν₀ is the threshold frequency, and it differs from metal to metal.

Materials respond differently: selenium is more sensitive than zinc or copper. One material also responds differently to different wavelengths: ultraviolet light ejects electrons from copper, but green or red light does not.

Above the threshold, emission begins at once, with no noticeable delay, even in very dim light. The delay is now known to be of the order of 10⁻⁹ s or less.

Summary of the observations: (i) the photocurrent is proportional to intensity; (ii) the saturation current is proportional to intensity while the stopping potential is independent of it; (iii) there is a threshold frequency below which nothing is emitted, and above it Kmax rises linearly with frequency but not with intensity; (iv) emission is instantaneous.