Explain the meaning of Lasers?

Lasers use these discrete energy levels in atoms or molecules to build a large population of atoms at an excited energy level that are ready and willing to jump down from a higher energy level, E₂, to a lower, more stable energy level, E₁. Each atom or molecule gives up a photon of the same energy. This shift in energy levels can be expressed as: E₂ - E₁ = DE = h f.

If a small number of photons of energy E are available for absorption, and many atoms wait in the excited state, E₂, then stimulated emission can occur where the photons are emitted in the same direction and phase, creating a coherent beam of light of a single frequency. More energy must be used to create the large number of electrons in energy level E₂ than can then be emitted as photons of laser light.

In the most common HeNe (helium and neon) laser, electrons are boosted from the He 1s² ground state to a metastable He 1s2s state at 20.61 eV through electron impacts. A large population of 5s Ne electrons are built up through collisions which then emit 632.8 nm laser light when they relax to the 18.70 eV Ne 3p electron state. Over time, these electrons find their way back to the Ne 2p ground state to start the trip again.