The photoelectric effect is the observation that many metals emit electrons when light falls upon them. Classical electromagnetic theory predicted that changing either the wavelength or the amplitude of the light would affect the emission of electrons. In addition, a dim light would be expected to result in a lag time between the initial shining of the light and the subsequent emission of an electron. Experimental results showed, however, that a high-frequency, low-intensity light produced electrons without the predicted lag time.
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Furthermore, the light used to dislodge electrons exhibited a threshold frequency, in which low-frequency light would not eject electrons from a metal regardless of its intensity or its duration. But high-frequency light would eject electrons, even if the intensity was low.