Stopping potential and the graphs

If the anode is made negative relative to the cathode, it repels the photoelectrons. The reverse voltage that just stops the fastest electrons (making the current zero) is the stopping potential V_s: e V_s = KE_max = hν − E_w.

Important graphs (for a given metal):

1. Photocurrent versus light intensity (ν > ν_c): a straight line through the origin; the current is proportional to the intensity. (For ν < ν_c the current is zero for every intensity.)
2. KE_max versus frequency: a straight line. It cuts the frequency axis at ν_c (KE = 0), its slope equals Planck's constant h, and its extension cuts the KE axis at −E_w. Different metals give parallel lines (same slope h), displaced along the frequency axis because their ν_c differ.
3. Photocurrent versus voltage: at positive voltage the current rises to a saturation value (all emitted electrons are collected). The saturation current is larger for greater intensity. At negative voltage the current falls to zero at −V_s; the value of V_s is the same for two beams of the same frequency, whatever their intensities.