Physics
Spontaneous and Stimulated Emission
Quick fact
A typical laser pointer uses stimulated emission to produce light billions of times more intense than the same number of excited atoms would emit spontaneously.
Why this is interesting
You've seen lasers cut through steel and project crisp images, yet a simple light bulb only glows warmly. What makes laser light so different?
Read the full explanation
Understanding Spontaneous and Stimulated Emission
Imagine a room full of excited atoms — each has extra energy and wants to relax. Spontaneous emission is like each atom randomly shouting out its energy as a photon, going in any direction and at any time. This is why a light bulb shines in all directions. Stimulated emission is different: when a passing photon of the right energy encounters an excited atom, it triggers that atom to emit an identical photon — same direction, same phase, same wavelength. It's like a coordinated wave in a stadium, where one person's action prompts another to join in exactly the same way. This process can amplify light if many atoms are excited, and it is the essence of how a laser works: Light Amplification by Stimulated Emission of Radiation.
A deeper explanation
At the quantum level, an atom in an excited state has a certain probability of decaying spontaneously (via Einstein's A coefficient) in a random direction and phase. Stimulated emission (Einstein's B coefficient) occurs when an external photon with energy equal to the energy difference interacts with the atom, inducing a transition that emits a second photon identical to the stimulating one. For amplification to happen, a population inversion must be maintained — more atoms in the excited state than the ground state — which is achieved using an energy pump (like a flashlamp or electrical discharge). The emitted photons then stimulate further emissions, creating a cascade that forms a coherent, monochromatic laser beam. This principle is not only key to lasers but also to optical amplifiers in fiber-optic communications and to understanding how masers and certain quantum devices operate.