eli5: How exactly do lasers work?


I understand that laser stands for Light Amplification by Stimulated Emission of Radiation. However, I don’t understand how radiation comes into play when trying to create a light source with no divergence.

In: 4

The big picture is that matter transitions from states of high energy to low energy, and when they do that, they release a photon of a specific wavelength.

But, if you can keep enough of those atoms at a high energy state, you can do something called “stimulated emission”, where a photon passing by your high energy atom can cause it to release its photon at exactly the same phase/frequency/direction as the first photon. Then, if you put some mirrors on either side, you can make both of those photons bounce back and forth between your sample.

Both of those photons can then cause stimulated emission on the other high energy atoms, and then you get a cascade of photons, all in exactly the same phase, and at exactly the same frequency and in exactly the same direction. Then, you poke a tiny hole in one of your mirrors, and you can point your laser at something.

Light is one of the many kinds of radiation. LASEL just doesn’t sound as good.

An excited atom wants to lose energy, and releases light when it does. In a laser, the excited atoms are created very carefully, so that more are excited than not. This is a special condition, which means that a photon is more likely to hit an excited atom than a normal atom.

When it hits the excited atom, it causes the atom to release another photon moving in the same direction, so now we have more photons. These continue hitting more excited atoms, releasing more and more photons and getting a good beam.

A laser can be “seeded” by having the first photon come from outside, or it can be self-starting, in which case two mirrors are used at either end to reflect light back and forth, allowing any “wrong” photons to quickly be lost while the “right” ones continue bouncing and generating more.

Light *is* radiation. All electromagnetic radiation is fundamentally the same thing – a wave of energy. That wave will have a specific wavelength, and the size of the wavelength determines what type of radiation it is.

Longer wavelength radiation gives us things like FM radio, or wi-fi.

Shorter wavelength radiation gives us X-Rays and other high-energy radiation.

In the middle is where you find visible light, from red to violet.

So the ‘radiation’ in LASER is just talking about the light that it produces. A plain old lightbulb also makes radiation, just not coherently.

There was a funny r/showerthought on here yesterday that said “A flashlight is a radiation gun with extremely low damage”, which is perfectly true.

Light is just a common name for electro-magnetic radiation.