Photoelectric Effect 

    The photoelectric effect is the name given to the observation that when light is shone onto a piece of metal, a small current flows through the metal. The light is giving its energy to electrons in the atoms of the metal and allowing them to move around, producing the current. However, not all colors of light affect metals in this way. No matter how bright a red light you have, it will not produce a current in a metal, but even a very dim blue light will result in a current flowing. The problem was that these results can't be explained if light is thought of as a wave. Waves can have any amount of energy you want - big waves have a lot of energy, small waves have very little. And if light is a wave, then the brightness of the light affects the amount of energy - the brighter the light, the bigger the wave, the more energy it has. The different colors of light are defined by the amount of energy they have. If all else is equal, blue light has more energy than red light with yellow light somewhere in between. But this means that if light is a wave, a dim blue light would have the same amount of energy as a very bright red light. And if this is the case, then why won't a bright red light produce a current in a piece of metal as well as a dim blue light? Einstein realized that the only way to explain the photoelectric effect was to say that instead of being a wave, as was generally accepted, light was actually made up of lots of small packets of energy called photons that behaved like particles. Einstein wasn't the first person to use the idea of photons, but he was the first to make it the starting point of an explanation rather than a convenient fiddle to explain away odd results.

Definition Provided by Year of Einstein 

Work functions 

 The work function is the minimum energy that must be given to an electron to liberate it from the surface of a particular substance. In the photoelectric effect, electron liberation is achieved by absorption of a photon. If the photon's energy is greater than the substance's work function, photoelectric emission occurs and the electron is liberated from the surface

Here is a table with a list of work functions for common materials.  Notice that work function is an electron Volt or eV..... so what is an electron volt?

Work functions

What is an Electron Volt 

   The electron volt (symbol eV; also written electronvolt) is a unit of energy equal to approximately 1.602×10^−19 joule (symbol J). By definition, it is the amount of energy gained by the charge of a single electron moved across an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb, 1 J/C) multiplied by the electron charge (1 e, or 1.602176565×10^−19 C). Therefore, one electron volt is equal to 1.602176565×10^−19 J.[3] Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge q has an energy E=qV after passing through the potential V; if q is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV.

Definition Provided by Wikipedia