LASER = Light Amplification by the Stimulated Emission of Radiation

Spontaneous emission - light is emitted by an atom when an electron drops from a higher energy level to a lower energy level.

Stimulated emission - a photon passing near an atom that has an electron in an excited state makes it more likely that the electron will drop to a lower state if the photon energy the electron emits is the same as that of the stimulating photon. In addition to having the same wavelength, the two photons are in phase and travel in the same direction.

....and they tell two friends, and they tell two friends, and so on, and so on, and so on...

For stimulated emission to work, the photons must encounter atoms with electrons in excited states. Consider how the helium-neon (He-Ne) laser works.

• A high voltage excites an electron in helium from the ground state to the first excited state. The two states are 20.61 eV apart.
  
  
• It turns out that two of the electron levels in neon are separated by an energy difference of 20.66 eV, almost identical to the difference of the two states in helium. Collisions between excited helium atoms and neon atoms can transfer energy to the neon atoms, exciting electrons in the neon atoms. This is very efficient because the energies are almost identical.
  
  
• This excited level in neon is a metastable state, which means the electron stays in that state for a while. This allows us to achieve a population inversion, in which there are more electrons in the excited state than in the lower states.
  
  
• The atoms are now primed for stimulated emission. A few electrons drop down to a level that is 1.96 eV lower in energy, (this energy corresponds to a wavelength of 633 nm). Having a few of these photons sets off a chain reaction via stimulated emission, leading to a beam of red light with a wavelength of 633 nm.
  
  

The green laser we sometimes use is also a helium-neon laser, emitting light at 543 nm. If 633 nm light corresponds to a difference in energy levels of 1.96 eV, the difference between energy levels for 543 nm light is:

1. larger than 1.96 eV
2. smaller than 1.96 eV

The green photons have a smaller wavelength, and therefore a larger frequency and energy, than do the red photons. Emitting a green photon requires a difference between energy levels of more than the 1.96 eV associated with the red photons.