Chapter 38 - Quantization - Conceptual Questions - Page 1151: 3

See the detailed answer below.

To understand how the graph would change according to classical physics, let's discuss the classical physics perspectives. According to the classical wave theory, the energy of light depends on its intensity, not on its frequency. According to classical physics, if you shine light of any frequency (even low-frequency light), the intensity should determine how much energy is transferred to the electrons. Thus, classical physics would predict that at any frequency, electrons could be ejected if the light is intense enough. The kinetic energy of the emitted electrons should depend on the intensity of the light, not on the frequency. So, under this classical assumption, for any frequency of light, you should observe a current. This means that the graph of current as a function of frequency should rise continuously from zero, with no threshold frequency $f_0$, since classical theory does not predict the necessity of a minimum frequency for electron emission. But​ If the light intensity remains constant, as in Figure 38.2, then the current should be constant, and hence, the graph would be a horizontal line starting from $f = 0$ Hz. See the graph below.