  It is well known that, for an incident light of not so high intensity and in
a certain range of frequency, the stopping voltage of photoelectric effect is
independent of the intensity but dependent on the frequency of the light, which
is described by the equation $V = h\nu /e - {W_0}/e$, where $V$ is the stopping
voltage, $h$ is the Planck constant, $\nu$ is the frequency of incident light,
$e$ is the electron charge, and $W_0$ is the work function. It means that the
larger the frequency of incident light, the higher the stopping voltage is.
However, the present experiment finds that for a non-monochromatic incident
light, the stopping voltage is not determined by the maximum frequency
component of the incident light, but by the maximum center frequency of all the
wave train components (with different center frequencies) involved in the
incident light, that is to say, in the photon energy expression $h\nu$,
physical quantity $\nu$ does not refer to the frequency of a monochromatic
light, but represents the center frequency of a wave train spectrum. The
spectral bandwidth of a wave train component can be as large as 122 nm in
visible and near-infrared region. This should arouse more attention in the
study of energy exchange between light and matter.
