Einstein won the Nobel Prize for Physics not for his work on relativity, but for explaining the photoelectric effect. He proposed that light is made up of packets of energy called photons.

The photoelectric effect occurs when light shines on a metal. Sometimes electrons are emitted.

Predictions of the wave theory of light:

• Light of any frequency will cause electrons to be emitted.
• The more intense the light the more kinetic energy the emitted electrons will have.

What actually happens:

• Light below a certain cutoff frequency, no matter how intense, will not cause any electrons to be emitted.
• Light above the cutoff frequency, even if it's not very intense, will always cause electrons to be emitted.
• Above the cutoff frequency, turning up the intensity produces more electrons but does not change the maximum kinetic energy of the electrons.

The explanation in terms of light being made up of photons:

• To eject one electron from the metal takes one photon.
  
  
• Electrons are bound to the metal by a binding energy we call the work function, W_o, which differs from metal to metal. If the photon energy is less than the work function, no electrons are emitted.
  
  
• The cutoff frequency f_o is where the photon energy hf_o = W_o
  
  
• Above the cutoff frequency the photons have more energy than what is needed to eject an electron. The extra energy shows up as the electron's kinetic energy:
  K_max = E - W_o = hf - W_o