In 1902, P. Lenard and others systematically studied the photoelectric effect in order to investigate the circumstances under which metal surfaces will emit electron (b-rays) when they are exposed to light.
Consider the case of sodium metal.
Observation #1: light of wavelength l=520 nm (green light) does not lead to emission of any electrons. Increasing the intensity of the light does not help!
Observation #2: light of wavelength l=450 nm (blue light) does lead to emission of "photoelectrons".
Observation #3: light of wavelength l=400 nm (violet light) leads to emission of "photoelectrons" with a higher kinetic energy than those which come off under irradiation at 450 nm.
Observation #4: increasing the intensity of light of wavelength l=400 nm (violet light) leads to emission of more "photoelectrons" (higher current) but with the same kinetic energy as for the lower intensity irradiation.
Here is some data for sodium:
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Let us convert the wavelength to frequency using n=c/l
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If we now look for a best-fit straight line through the data ( y= mx+b), we get the following:
The straight line equation we get has the form
y= mx+b
where here
y is KE
x is 
m=6.59x10-34 J-s
(note this is Planck's constant, h)
b=-4.39x10-19J (we'll let W=-b)
So we get Einstein's equation for the photoelectric effect:
KE = h-W
W (the metal's work function) is the minimum energy photon required to eject an electron
An alternative way to write this equation is
h=
W + KE
which expresses the fact that the energy of the photon is partitioned between the minimum needed to eject the electron and the remainder goes into kinetic energy.
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