# Q & A: Can laser light be made using a filter?

Q:
Greetings, and thanks in forward for answer on my question. My question is related to monochromatic light and the concept of a laser. If I use a prism to get a rainbow, then use an appropriate filter to get only (lets say) the color red, I should get more-or-less monochromatic light. After that process, would I be able to implement a photomultiplicator, focus the beam with some lenses, and get some laser beam (monochromatic light)?
- Dragan (age 18)
Serbia
A:

Hi Dragan

That is a cool idea, but it won't work. There are some concepts about monochromatic light and lasers I will explain so you can see why.

Being monochromatic isn't the only thing that makes laser light special. Using your filter scheme you can theoretically get the light to be as monochromatic as you like, though the more monochromatic you try to make it, the less light will make it through the filter. The other thing that makes a laser special is that it emits light with a constant phase.

Let's talk about phase, in case it's an unfamiliar term. Waves oscillate up and down; they have peaks and troughs. The phase of a wave is a measure of where the wave is in its oscillation. Usually, we say that there are 360 degrees in one oscillation, so for example, if we mark one peak as 0 degrees phase, then in 180 degrees (halfway through the oscillation) we'll be at a trough, and after 360 degrees we'll be at another peak.

To see how a laser maintains a constant phase, let’s discuss how one is made. A laser requires a cavity, which is formed with highly reflective mirrors that trap the light between them. Between the mirrors there is some type of medium - for example a gas or a crystal - that absorbs light at one wavelength and emits it at another. The laser cavity allows the atoms in the medium to simultaneously be placed in a higher energy level when it absorbs the light, and to emit at a lower wavelength when it falls to another, lower energy level.

The cavity in a laser keeps a well-defined phase, for one thing because it has sharp boundaries- the waves must have zero amplitude at the mirrors- and for another because once the medium has most of its atoms placed in a higher energy level, a single photon of light passing through stimulates the other atoms to all emit photons that are exactly the same, including having the same phase. So from a laser, you get light that is one long, continuous wave. It's known as coherent light. And that is super cool, because then we can see the laser light interfere (interference is when the peaks and troughs of waves line up in different places and cause the amplitudes to either increase or decrease), which we definitely don't see in typical light sources. The filters aren't capable of making the light coherent, because they just cut out photons from a source that aren't at the right wavelength, and those photons presumably aren't coherent unless they're from a laser. Note that the sharp boundaries of the laser cavity allow only certain wavelengths of light to exist in the space between, which is why the light is highly monochromatic as well as coherent.

- Courtney K

(published on 03/06/2016)