What is Laser and how do it work ?

Now suppose also that we could maintain our atoms in this state for a little while so they didn't automatically jump back down to their ground state (a temporarily excited condition known as a meta-stable state).

Then we'd find something really interesting. If we fired a photon with just the right energy through our bunch of atoms, we'd cause one of the excited electrons to jump back down to its ground state, giving off both the photon we fired in and the photon produced by the electron's change of state.

Because we're stimulating atoms to get radiation out of them, this process is called stimulated emission.

We get two photons out after putting one photon in, effectively doubling our light and amplifying it (increasing it).

These two photons can stimulate other atoms to give off more photons, so, pretty soon, we get a cascade of photons—a chain reaction—throwing out a brilliant beam of pure, coherent laser light. What we've done here is amplify light using stimulated emission of radiation—and that's how a laser gets its name.

If that's how lasers make light, why do they make a single color and a coherent beam? It boils down to the idea that energy can only exist in fixed packets, each of which is called a quantum.

It's a bit like money. You can only have money in multiples of the most basic unit of your currency, which might be a cent, penny, rupee, or whatever.

You can't have a tenth of a cent or a twentieth of a rupee, but you can have 10 cents or 20 rupees. The same is true of energy, and it's particularly noticeable inside atoms.

Like the rungs on a ladder, the energy levels in atoms are in fixed places, with gaps in between them.

You can't put your foot anywhere on a ladder, only on the rungs; and in exactly the same way, you can only move electrons in atoms between the fixed energy levels.

To make an electron jump from a lower to a higher level, you have to feed in a precise amount (quantum) of energy, equal to the difference between the two energy levels.

When electrons flip back down from their excited to their ground state, they give out the same, precise amount of energy, which takes the form of a photon of light of a particular color.

Stimulated emission in lasers makes electrons produce a cascade of identical photons—identical in energy, frequency, wavelength—and that's why laser light is monochromatic.

The photons produced are equivalent to waves of light whose crests and troughs line up (in other words, they are "in phase")—and that's what makes laser light coherent.

“Laser” is an acronym for light amplification by stimulated emission of radiation. A laser is created when the electrons in atoms in special glasses, crystals, or gases absorb energy from an electrical current or another laser and become “excited.”

“Laser” is an acronym for light amplification by stimulated emission of radiation.

A laser is created when the electrons in atoms in special glasses, crystals, or gases absorb energy from an electrical current or another laser and become “excited.”

The excited electrons move from a lower-energy orbit to a higher-energy orbit around the atom’s nucleus. When they return to their normal or “ground” state, the electrons emit photons (particles of light).

These photons are all at the same wavelength and are “coherent,” meaning the crests and troughs of the light waves are all in lockstep.

In contrast, ordinary visible light comprises multiple wavelengths and is not coherent.

Since we can excite many different kinds of atoms in many different ways, we can (theoretically) make many different kinds of lasers.

Laser light is different from normal light in other ways as well.

First, its light contains only one wavelength (one specific color).

The particular wavelength of light is determined by the amount of energy released when the excited electron drops to a lower orbit. Second, laser light is directional.

Whereas a laser generates a very tight beam, a flashlight produces light that is diffuse. Because laser light is coherent, it stays focused for vast distances, even to the moon and back.