# Lasers and the electromagnetic spectrum

The color of light you choose will have a big effect on what your laser beam can do. Let’s discuss various colors of light, both visible and invisible, to get an overview of what each is good for and what limitations each may have. After all, choosing the right color is the key to really making your laser shine.

## The Nature of Light

You knew we had to go through this, didn’t you? Lasers are made of light, so we need some understanding of some of the basic physics of light in order to really grok lasers. Light is made when an accelerating charged particle shakes off some of its electric or magnetic field. These bits of free field naturally end up re-making themselves in such a way that they fly off and propagate through space as a wave. All the stuff that you see is actually wiggling bits of electric and magnetic fields continually re-making themselves almost like they were dragging themselves along by their bootstraps.

As you might guess from the name, these wiggling waves zip around at the speed of light. But we need to be a wee bit careful here, because light goes a bit slower through a transparent medium like water or glass or even air than it does through the vacuum of space. In air, light moves so close to the speed of light in vacuum that we can usually ignore the difference. But in transparent condensed materials like water or glass, the light might only be going ${\displaystyle {\frac {2}{3}}}$ or ${\displaystyle {\frac {1}{2}}}$ or even less of the speed in vacuum.

As a wave, light has a number of properties that can be used to describe it. One is the wavelength - how far it is from the crest of the wave to the crest of the next cycle. This is closely related to the frequency - if you are staying in one spot, how many times does the light wave go up and down in a given amount of time. If you multiply the frequency and wavelength together, you will get the speed of the wave. So with a bit of algebra and knowing that light moves at the speed of light, if you know the wavelength you can find the frequency, and vice versa. When light goes from one material to another, the frequency stays the same but the wavelength changes. Many people characterize light by its wavelength - almost always they are talking about its vacuum wavelength, not the wavelength it has in any particular medium.

The only other physical things that describe a pure wave of light are its direction of travel and the direction the light is vibrating. This latter direction is called the polarization of the light and is always perpendicular to the direction of travel. All other physical properties can be found by knowing the frequency, direction, and polarization. [1] [2] [3] [4]

In particular, the color of the light depends on its frequency. High frequency (and thus short wavelength) visible light looks violet, and as we go to lower frequencies (and thus longer wavelengths) the light goes across the rainbow of colors from violet to blue to green to yellow to orange until the lowest frequency light we can see is red. But it doesn’t stop there. There are a great many invisible colors of light that we can’t see, which we will describe below.

One other characteristic commonly used to describe very high frequency light is the energy-per-photon. A photon is the smallest possible amount of vibration possible, as dictated by quantum mechanics. The energy in this minimum amount of electromagnetic radiation is the frequency of the light times a constant of nature called Planck’s constant. [5] Again, knowing any one of the frequency, vacuum wavelength, or energy-per-photon lets you find all the others. Because higher frequency light has more energy-per-photon, a pulse of a given energy will have fewer photons in it the higher the frequency of the light. But having a higher energy-per-photon does not mean you automatically get more energy in your beam!

Radio waves are essentially useless for lasers. Sure, they get through the air okay, but diffraction makes them almost impossible to focus enough to cause damage. So without further ado, we’ll move on.

 Color Frequency Wavelength Energy Radio 30 Hz - 30 MHz 10,000 km - 1 m < 1.25 μeV

## Microwaves

Microwaves, at least, focus better than radio waves. They still don’t focus well enough to make practical weapons - at least not if your intent is cooking or burning or blasting your enemy. They are useful at projecting destructive currents into electronic circuits. Such high power microwave devices operating in a counter-electronics role are usually considered a different class of weapon than lasers, so we will leave them for now.

 Color Frequency Wavelength Energy Microwave 30 MHz - 300 GHz 1 m - 1 mm 1.25 μeV - 1.25 meV

## Terahertz waves

Terahertz waves are all the rage these days for remote scanning and a new window for material spectroscopy. Unfortunately, they are absorbed by the air within a few tens of meters, so they are a poor choice for a weapon.

 Color Frequency Wavelength Energy Terahertz wave 300 GHz - 10 THz 1 mm - 30 μm 1.25 meV - 40 meV

## Far infrared

Far infrared is a sort of orphan band of the electromagnetic spectrum, because it is hard for us to get sources and detectors in this range. As a result, we don’t have a lot of experience with what it can do. In principle, your high-tech sci-fi society could be able to make fiercely high power far infrared lasers. But they’re still not a good choice for a weapon because the diffraction limit makes them hard to focus to damaging intensities without huge focal apertures and they’d have bad issues with cascade breakdown of the air.

 Color Frequency Wavelength Energy Far infrared 10 THz - 20 THz 30 μm - 15 μm 40 meV - 80 meV

## Long-wave infrared

This is the electromagnetic band where most thermal radiation from room-temperature and body-temperature objects occur. When someone is seeing heat with infrared vision, this is the color they are seeing with. The atmosphere is quite transparent to long-wave infrared radiation. Unfortunately, these long wavelengths are still difficult to focus at any useful distance, and they tend to cause cascade breakdown in the air when high powers are put into tight focal spots. The main reason they pop up in talking about lasers is that one of the first kinds of high power lasers, and still one of the cheapest and simplest to build, is the carbon dioxide laser which operates in the long-wave infrared.

 Color Frequency Wavelength Energy Long-wave infrared 20 THz - 38 THz 30 μm - 15 μm 80 meV - 160 meV

## Mid-wave infrared

A lot of the mid-wave infrared spectrum is absorbed quickly by air. However, there is a “window” between 3.5 and 4 μm where the light can get through. This window was investigated by early laser weapon designers, using chemical deuterium fluoride lasers. Chemical laser weapons were nasty, toxin-spewing, noisy monstrosities of machines with abysmal beam quality and long logistics chains to supply their highly toxic, corrosive, flammable, and explosive chemicals. And even when they were the only game in town, the deuterium fluoride laser was replaced as soon as they could by chemical oxygen iodine lasers that at least operated in the near infrared and so could be focused three times as far. Today we have far better choices, so don’t expect mid-wave infrared lasers to get much love.

 Color Frequency Wavelength Energy Mid-wave infrared 38 THz - 100 THz 8 μm - 3 μm 160 meV - 410 meV

## Short-wave infrared

Short-wave infrared is a good choice when you are looking for a color of light that focuses well, can get through air, can maintain a tight focus without two-photon ionization messing it up, and that won’t pose a severe blinding hazard to anyone nearby. It is the shortest wavelength (and thus longest ranged) color of infrared that is eye-safe. You won’t get eye-safe colors again until you are up into the ultraviolet.

Some modern lasers can output high power short-wave infrared beams, primarily fiber lasers.

 Color Frequency Wavelength Energy Short-wave infrared 100 THz - 215 THz 3 μm - 1.4 μm 410 meV - 900 meV

## Near infrared

This is the color that almost all modern combat lasers operate. The air is nicely transparent to light at this color, the beams focus well enough, and you can get crazy high powers out of fiber lasers these days. Sure - look at the thing they are shooting and you might go blind, but there are bigger hazards in the military.

 Color Frequency Wavelength Energy Near infrared 215 THz - 430 THz 1.4 μm - 0.7 μm 900 meV - 1.8 eV

## Visible

Yeah baby! Now we’re talkin’! Flashing beams lighting up the sky for stunning visual effects, strobing flashes where the beams hit. These are the beams you can really see! Except for the minor detail that if you ever do actually see one in person, there’s a good chance you won’t ever see anything again. But we’ll ignore that for the sake of a good special effects loaded sci-fi extravaganza.

The air is very transparent to visible light. It turns out that water is also at around its optimal transparency to visible light in the green, blue, and violet colors, so lasers of these colors might be the choice for underwater combat (or at least, underwater combat where you are close enough to see your enemy). If you want maximum range in air without two photon absorption making your life miserable when your beam reaches close focus, visible light is the color choice for you.

Visible light is where you start having significant losses due to Rayleigh scattering for light going all the way through an Earth-like atmosphere. Shorter wavelengths (the closer to violet) are scattered more than longer wavelengths (closer to red). But still, the shorter wavelengths focus better because of diffraction. When shooting straight down through Earth’s atmosphere, you still get the best laser intensity with violet light (even though your beam will have less energy, that energy will still be more concentrated). But as you shoot at more and more of an angle, the best choice goes toward longer and longer wavelengths. And for alien planets, all bets are off.

 Color Frequency Wavelength Energy Red 430 THz - 480 THz 0.7 μm - 0.62 μm 1.8 eV - 2 eV Orange 480 THz - 510 THz 0.62 μm - 0.59 μm 2 eV - 2.1 eV Yellow 510 THz - 530 THz 0.59 μm - 0.57 μm 2.1 eV - 2.2 eV Green 530 THz - 610 THz 0.57 μm - 0.49 μm 2.2 eV - 2.5 eV Blue 610 THz - 670 THz 0.49 μm - 0.45 μm 2.5 eV - 2.7 eV Violet 670 THz - 750 THz 0.45 μm - 0.4 μm 2.7 eV - 3.1 eV

## Near ultraviolet

This region of the spectrum is made up of invisible colors with shorter wavelengths than we can see that can still go through air. But although these colors can get through sea level air, ozone in the upper atmosphere does a pretty good job of absorbing ultraviolet light with wavelengths shorter than 0.34 μm. So if you want to use your laser to shoot things on the ground from your spacecraft, choose a wavelength longer than 0.34 μm. Ultraviolet light is scattered more by air than visible light, which makes it more favorable to use visible light for shooting things on Earth from a spacecraft. But on an alien planet, the scales may tip in favor of ultraviolet. And if you are shooting down within the atmosphere, the optimum color depends on the range to the target. And there is also the issue that high powered ultraviolet pulses are likely to be absorbed due to two-photon absorption before they get to their target.

Ultraviolet light is eye-safe at wavelengths shorter than 0.35 μm. Some ultraviolet-A colored light can get through window glass, but ultraviolet-B or C cannot. So if you are shooting your laser gun at someone on the other side of a window, choose ultraviolet-A or longer wavelengths.

 Color Frequency Wavelength Energy Ultraviolet-A 750 THz - 950 THz 0.4 μm - 0.315 μm 3.1 eV - 3.9 eV Ultraviolet-B 950 THz - 1.1 PHz 0.315 μm - 0.28 μm 3.9 eV - 4.4 eV Ultraviolet-C 1.1 PHz - 1.5 PHz 0.28 μm - 0.2 μm 4.4 eV - 6.2 eV

## Vacuum ultraviolet

Wavelengths shorter than 0.2 μm can’t go through oxygen. Hence, these wavelengths are called “vacuum” frequencies because they can’t go through air, only vacuum. Other atmospheres may let through slightly shorter wavelengths, but not by much. Anything shorter than 0.1 μm won’t be able to go through any matter (except perhaps a pure helium atmosphere - which can let stuff through down to 0.05 μm).

Still, as much as vacuum ultraviolet can’t be used in air, it focuses really well in vacuum. If you can make it in your laser, and if you can focus it, it’s a great choice for shooting things in space. In today’s world, our ultraviolet optics are not great and a lot of the light will get absorbed when we want it to be reflected. But a science fiction setting might have solved this, allowing their spaceships to mount devastating long range ultraviolet lasers.

 Color Frequency Wavelength Energy Vacuum ultraviolet 1.5 PHz - 30 PHz 0.2 μm - 10 nm 6.2 eV - 125 eV

## Soft x-ray

The boundary between ultraviolet and x-ray is kind of fuzzy - there is no strict line with ultraviolet on one side and x-rays on the other. The lower energy-per-photon sorts of x-rays act a lot like vacuum ultraviolet. Sure, they focus better because of their shorter wavelengths but they still get almost immediately absorbed by air. The best way we can find to focus soft x-rays is by using a complicated grazing incidence mirror.

 Color Frequency Wavelength Energy Soft x-ray 30 PHz - 3 EHz 10 nm - 0.1 nm 125 eV - 12.5 keV

## Hard x-rays

At higher energies-per-photon, x-rays start being able to go some distance through matter before they can be absorbed. There is no hard and fast rule on what is a soft x-ray and what is a hard x-ray. But these little photons can zip through tens of meters of air, or stand a reasonable chance of making it through a person’s body. This still makes them impractical for use as a weapon in an atmosphere, however. But they can really allow your spacecraft to reach out and touch someone if you can somehow figure out a way to make and focus these little guys. Grazing incidence telescopes can work, but become more and more difficult the more energetic the x-rays get.

If you shoot a hard x-ray beam through an enemy spacecraft, expect the people that were in the compartments the beam penetrated to die from radiation poisoning if the blast and heat don’t get them.

 Color Frequency Wavelength Energy Hard x-ray 3 EHz - 30 EHz 0.1 nm - 0.01 nm 12.5 keV - 125 keV

## Gamma rays

The highest energy photons are called gamma rays. As with a lot of this high energy-per-photon light, there isn’t really a sharp distinction between high energy x-rays and low energy gamma rays. Gamma rays can get through several hundred meters of air. But don’t use a gamma ray laser if you are in air - the scattered radiation will come back to give you radiation sickness. Much like with hard x-ray, anyone even near where a gamma ray laser goes through matter will be dosed with dangerous levels of ionizing radiation. Also like hard x-rays, we have no idea how we would focus gamma rays.

 Color Frequency Wavelength Energy Gamma ray > 30 EHz < 0.01 nm > 125 keV

## Credit

Author: Luke Campbell

## References

1. J. D. Jackson, “Classical Electrodynamics”, John Wiley & Sons, New York (1975)
2. David J. Griffiths, “Introduction to Electrodynamics”, Prentice Hall, Englewood Cliffs, New Jersey (1989)
3. Halliday, Resnick, and Walker, “Fundamentals of Physics”, John Wiley & Sons (2005)
4. Douglas C. Giancoli, “Physics for Scientists and Engineers, Second Edition”, Prentice Hall, Englewood Cliffs, New Jersey (1988)
5. Hans C. Ohanian, “Principles of Qunatum Mechanics”, Prentice Hall, Englewood Cliffs, New Jersey (1990)