Einstein and the Photoelectric Effect

	Hit any materials hard enough with some light and you will kick out some electrons. That phenomena is known as the photoelectric effect, and electrons emitted in this manner may be referred to as "photoelectrons".
		The photoelectric effect was first observed by Heinrich Hertz in 1887; the phenomenon was also known as the "Hertz effect" then.
		The strange thing was that "hard enough" did not mean, as one would have expected, plenty of light or, more precisely, large amplitudes of the light waves. Instead it meant high enough frequency. No amount of light with a frequency just a little bit too small could produce any photoelectrons, while light with a frequency just above a critical values did - even at low intensities. It was just how many electrons were released per second that depended on the intensity but not if electrons were kicked out.
		There is simply no way to understand this observation employing only "wave" optics. You need the photon concept.
	Much later (in 1921) the Compton effect once more demonstrated that light with high frequencies (also called X-rays or g radiation) shows distinct particle properties when encountering atoms.
		The Hertz effect had to wait for 18 years before an explanation was found in 1905 by Albert Einstein. He solved the apparent paradox by introducing what we now call the "photon", a discrete quanta of light, somewhat particle-like, instead of a continuous wave.
		Einstein based his approach on Max Planck's theory of black-body radiation. He postulated that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck's constant. A photon above a threshold frequency then has the required energy to eject a single electron, creating the observed effect

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5.1.1 What is Light?