I would agree with David, who looked "at the light intensity from an
incandescent bulb". The expected frequency of light intensity should
be the same as power P(t)=R*I(t)*I(t). If I(t) changes at 60 Hz then
P(t) changes at 120 Hz (sin^2 curve) because R is essentially constant.
R is determined by the mean temperature.
None of the replies convinced me that the observed 60 Hz, instead of
the expected 120 Hz, is reasonable. Are you sure, David, that your
detector is responding to light? Perhaps it acts as an antenna and
is responding to I(t).