When UV light hits a metal surface, it causes an emission of electrons.

According to Classical Electromagnetic Theory, the ‘photoelectric effect‘ can be attributed to the transfer of energy from the light to an electron in the metal.  Viewed from this perspective, any alteration in amplitude or wavelength of light ought to induce changes in the rate of emission of electrons from the metal.  According to this theory, a sufficiently dim light would be expected to show a lag time between the initial shining of its light and the subsequent emission of an electron.  However, the experimental results did not correlate with either of the two predictions made by this theory.

Instead, electrons are only dislodged by the photoelectric effect if light reaches or exceeds a threshold frequency, below which no electrons can be emitted from the metal regardless of the amplitude and temporal length of exposure of light.  To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that

A beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (or photons), each with energy hf.

This idea shed revolutionary new light on Max Planck’s previous discovery of the Planck relation, that is

where the photon energy E is proportionally linked to the frequency f.

The factor h is now known as the Planck constant.