What is the difference between spontaneous and stimulated emission

The main difference is that spontaneous emission is a random process where an excited electron falls to a lower energy state on its own, releasing a single photon in a random direction. Stimulated emission is a triggered process where an incoming photon causes an already-excited electron to fall, releasing a second photon that is a perfect copy (same direction, phase, and energy) of the first.

This difference is the reason why light from a light bulb (spontaneous) is jumbled and diffuse, while light from a laser (stimulated) is intense, focused, and coherent.

Spontaneous Emission 

This is the most common way light is produced in nature.

1. Excitation: An electron in an atom absorbs energy (from heat, a collision, or other light) and jumps to a higher, unstable energy level.
2. Spontaneous Decay: After a very short, random amount of time, the electron "spontaneously" falls back to its stable, lower energy level.
3. Photon Release: To conserve energy, the electron releases the excess energy as a single particle of light, a photon.
4. Result: The emitted photon has a random direction and phase.³ This is why sources like the sun or a light bulb send light out in all directions (incoherently).

Stimulated Emission 

This is the special process that makes lasers work (it's the "S" and "E" in LASER: Light Amplification by Stimulated Emission of Radiation).

1. Excitation: Just like before, an electron is first excited to a higher energy level.
2. Triggering Photon:Before the electron can fall on its own, it is "hit" by a passing photon. This trigger photon must have an energy that exactly matches the energy the electron is waiting to release.
3. Forced Decay: This interaction stimulates (or forces) the excited electron to fall back to its lower energy level immediately.
4. Photon Cloning: When the electron falls, it releases its own photon. This new photon is an identical clone of the trigger photon. It has the:
   • Same Energy (same color/wavelength)
   • Same Direction of travel
   • Same Phase (the light waves line up perfectly)
5. Result: You now have two identical, coherent photons where you started with one. This process can cascade, creating an amplified, perfectly aligned beam of coherent light.