A question about lasers

[Stofsk]

Let's say we've got your typical sci-fi laser gun. I have two questions:

1. Does the laser beam make a sound, or is it soundless? If it makes a sound, what kind would it make? A gun-like 'crack!', or a 'pew-pew!', a phaser-like 'hum' or what? If it doesn't make a sound, why not?

2. Is the beam visible or invisible? Obviously if it is a vacuum, the answer would be a quick no. But I'm talking about using a laser gun in an atmosphere. Would the beam be visible or not? If not, why not? Can it be made visible? If it is visible, can it be made invisible? If it can be visible, what kind of colour would the beam be?

[Dakarne]

Does the laser beam make a sound, or is it soundless?

Realistic Laser: Soundless, possibly a click from being switched on, and a small humming from electricity

If it makes a sound, what kind would it make?

Click and barely audible hum.

If it doesn't make a sound, why not?

Because it is a concentrated beam of light, basically a focused lightbulb.

Is the beam visible or invisible? Obviously if it is a vacuum, the answer would be a quick no. But I'm talking about using a laser gun in an atmosphere.

If it's an invisible atmosphere (Air), no visibility... in smoke and mist it would be visible though.

That answer your question?

[Il Saggiatore]

Let's say we've got your typical sci-fi laser gun. I have two questions:

1. Does the laser beam make a sound, or is it soundless? If it makes a sound, what kind would it make? A gun-like 'crack!', or a 'pew-pew!', a phaser-like 'hum' or what? If it doesn't make a sound, why not?

If you are referring to the beam itself, and not the gun, then it depends on its effect on the atmosphere.
If the atmosphere can scatter a fraction of the beam, and the amount of power dissipated into the atmosphere as heat is large enough, then you might hear a sound.
What it would sound like in this case, I am not sure; maybe a hum or a whip-crack-like sound (switch on the laser, beam heats quickly a portion of the atmosphere, and you have a sort of shockwave).

2. Is the beam visible or invisible? Obviously if it is a vacuum, the answer would be a quick no. But I'm talking about using a laser gun in an atmosphere. Would the beam be visible or not? If not, why not? Can it be made visible? If it is visible, can it be made invisible? If it can be visible, what kind of colour would the beam be?

There is no general case.
It all depends on how the beam is scattered by the atmosphere and what produces the beam.
In particular, the scattering depends on the frequency.

[Admiral Valdemar]

Let's say we've got your typical sci-fi laser gun. I have two questions:

1. Does the laser beam make a sound, or is it soundless? If it makes a sound, what kind would it make? A gun-like 'crack!', or a 'pew-pew!', a phaser-like 'hum' or what? If it doesn't make a sound, why not?

What power is it? Does it warm your skin as if in a nice sunbeam, or does it vape steel? If the latter, it makes a colossal crack louder than any gun, more like a lightning strike. The capacitor bank would make a hum or whine as it charged and was ready.

2. Is the beam visible or invisible? Obviously if it is a vacuum, the answer would be a quick no. But I'm talking about using a laser gun in an atmosphere. Would the beam be visible or not? If not, why not? Can it be made visible? If it is visible, can it be made invisible? If it can be visible, what kind of colour would the beam be?

What frequency laser is it? Is it IR, blue/green, UV, X-ray, gamma? How powerful is it? You can't just ask this vague question, it's like asking how fast a car can accelerate and what colour it is. Totally meaningless. If in an atmosphere, you'll want something like an IR one or, better yet, a maser (microwave laser, atmosphere doesn't affect it). Both would be invisible to the naked eye bar maybe a heat-haze forming along the beam. Blue/green lasers, unlike red ones, will be readily visible in broad daylight even as laser pointers, letalone MW scale weapons. X-rays will ionise the atmosphere producing a lightning bolt (blue/purple colour for our atmosphere). In space, without some matter in the way of the beam, you won't see anything.

[tharkûn]

1. Does the laser beam make a sound, or is it soundless? If it makes a sound, what kind would it make? A gun-like 'crack!', or a 'pew-pew!', a phaser-like 'hum' or what? If it doesn't make a sound, why not?

Depends on the type of laser and the conditions under which it fires. Cracks can come from two major sources: the target and the air the beam travels in. The former happens when you heat the target enough that it attempts to change volume due to thermal expansion/contraction and is prevented from doing so until the strain reaches a critical point and cracks propogate on the surface. The latter comes about because some of the energy is dumped into the air (or other medium) and in some cases it superheats the air sufficiently to behave in a manner similar to lightening.

Mechanical noises result merely from the movement of various parts of the laser. Theoreticly you could build a quiet high powered laser, realisticly you will always have something that vibrates.

2. Is the beam visible or invisible? Obviously if it is a vacuum, the answer would be a quick no. But I'm talking about using a laser gun in an atmosphere. Would the beam be visible or not? If not, why not? Can it be made visible? If it is visible, can it be made invisible? If it can be visible, what kind of colour would the beam be?

Many variables. The most common way we "see" laser beams is for some of the light to be scattered off particulent matter suspended in the path of the laser (dust is really good at this). Even in a pristine argon box you can still see the laser under some circumstances. Part of it gets back to waste heat, if you are bleeding enough energy into the surrounding air quick enough you will heat it enough that some thermal radiation will come off in the visible spectrum (it is almost impossible not to light up the IR). At some wavelengths (very short) you might get the odd electronic transition which gives off Stokes or anti-Stokes in the visible spectrum (high energy photon goes into particle and either comes back with less or more more respectively - the particle will then relax and emit or absorb a photon corresponding to the energy gap). In the cases where you can't see the beam, nothing is reflecting a wavelength you can see nor refracting anything visible into your eye.

Making a beam visible is trivial - spray with a fine mist of almost anything. If you want to make the beam invisible you need to use a frequency that doesn't give rise to photons you can see. That means avoiding all the vibrational, rotational, and electronic excitation energies. That means you have to write off most of the RF, most microwave, a good portion of the IR, virtually all the VUV, and anything X-ray and up (those ionize atoms which give rise to a whole slew of subsidiary reactions) in earth's normal atmosphere (you can work around this, but not at any impressive type of power or energy). After that you want a tight beam and one that is optimized in pulse length (too long and you will start shedding too much heat in one place, too little and you won't be able to shed waste heat fast enough).

Color is whatever you want it to be when dealing with reflections play around with frequency and multiple sources. When dealing with waste heat the color will be dictated by black body temperatures (as an approximation) - the hotter you are running the more blue/white and less red you will see. Electronic transitions are dependent on what you are irradiating in the medium.